THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


The  RALPH  D.  REED  LIBRARY 


DEPARTMENT  OF  GEOLOGY 


A  LIBRARY  OF 
UNIVERSAL  LITERATURE 

IN    F  O  U R    PARTS 

Comprising  Science,  Biography,  Fiction 
and  the  Great  Orations 

PART   ONE— SCIENCE 

Text-Book  of   Geology 

(PART    ONE) 


BY 

SIR    ARCHIBALD    GEIKIE,  F.R.S. 


NEW    YORK 

P.  F.  COLLIER  AND  SON 
•M  C  MI  • 

29 


PRESS  OF 
P.  F.  COLLIER  &SON 


ALL    RIGHTS    RESERVED 


Geology 
Library 


A   LIBRARY   OF 
UNIVERSAL    LITERATURE 


SCIENCE 

VOLUME  TWENTY-NINE 


BOARD  OF  EDITORS 


SCIENCE 

ANGELO    HEILPRIN,  author  of   "The  Earth  and  Its  Story,"  etc.; 

Curator  Academy  of  Natural  Sciences  of  Philadelphia. 
JOSEPH   TORRE  Y,  JR.,  Ph.D.,  Instructor  in  Chemistry  in   Harvard 

University. 
RAY  STANNARD  BAKER,  A.B.,  author  of  "The  New  Prosperity," 

etc.;  Associate  Editor  of  McClure's  Magazine. 

BIOGRAPHY 

MAYO  W.  HAZELTINE,  A.M.,  author  of  "Chats  About  Books,"  etc.; 

Literary  Editor  of  the  New  York  Sun. 
JULIAN   HAWTHORNE,  author  of  "Nathaniel   Hawthorne  and   His 

Wife,"  "History  of  the  United  States,"  etc. 
CHARLES  G.  D.  ROBERTS,  A.B.,   A.M.,  author  of  "A   History  of 

Canada";    late  Professor  of  English  and    French    Literature, 

King's  College. 

FICTION 
RICHARD  HENRY  STODDARD,  author  of  "The  King's  Bell,"  etc.; 

Literary  Editor  of  the  New  York  Mail  and  Express. 
HENRY  VAN  DYKE,  D.D.,  LL.D.,  author  of  "Little  Rivers,"  etc.; 

Professor  of  English  Literature  at  Princeton  University. 
THOMAS  NELSON  PAGE,  LL.D.,  Litt.D.,  author  of  "Red  Rock,"  etc. 

ORATIONS 

HON.  HENRY  CABOT  LODGE,  A.B.,  LL.B.,  author  of  "Life  of  Daniel 
Webster,"  etc.;  U.  S.  Senator  from  Massachusetts. 

HON.  JOHN  R.  PROCTOR,  President  U.  S.  Civil  Service  Commission. 

MORRIS  HICKEY  MORGAN,  Ph.D.,  LL.D.,  Professor  in  Latin,  Har- 
vard University. 


SIR  ARCHIBALD  GEIKIE 

SIR  ARCHIBALD  GEIKIE  was  born  at  Edinburgh  on  De- 
cember 28,  1835.  He  is  of  French  descent  on  the  maternal 
side.  He  was  educated  at  the  High  School  and  University 
of  his  native  city,  but  has  since  been  made  a  D.C.L.  by 
Oxford,  a  D.Sc.  by  Cambridge  and  Dublin,  and  LL.D. 
by  Edinburgh  and  St.  Andrews.  He  entered  the  Geolog- 
ical Survey  in  1855,  and  became  successively  Director  of 
the  Geological  Survey  of  Scotland,  Murchison  Professor 
of  Geology  and  Mineralogy  iu  Edinburgh  University, 
Director  of  the  Museum  of  Practical  Geology  in  Jermyn 
Street,  London,  Foreign  Secretary  of  the  Eoyal  Society, 
President  of  the  Geological  Society  and  President  of  the 
British  Association.  He  is  a  Correspondent  of  the  Institute 
of  France,  of  the  Lincei,  Eome,  of  the  Academies  of  Berlin, 
Vienna,  Belgium,  Stockholm,  Turin,  Munich,  Christiania, 
Gottingen,  and  so  forth.  He  has  been  the  recipient  of 
innumerable  medals.  Among  his  numerous  publications 
we  have  selected  for  reproduction  the  "Text-Book  of  Geol- 
ogy," which  has  been  translated  into  almost  every  Euro- 
pean language.  ^ 


734004 


PREFACE  TO  THE  THIRD  EDITION 

THE  present  edition  of  this  Text-book  has  been  entirely 
revised,  and  in  some  portions  recast  or  rewritten,  so  as  to 
bring  it  abreast  of  the  continuous  advance  of  Geological 
Science.  The  additions  made  to  the  text,  which  extend  to 
every  branch  of  the  subject,  increase  the  volumes  by  about 
270  pages.  Care  has  been  taken  to  preserve  a  characteristic 
feature  of  former  editions  by  inserting  references  to  the 
more  important  memoirs  and  papers,  where  the  student  will 
find  fuller  information  than  can  be  given  in  a  Text-book. 

While  the  book  was  passing  through  the  press  I  received 
from  my  friend  Professor  Zirkel  the  first  volume  of  the  new 
edition  of  his  great  text-book  of  Petrography,  but  too  late 
to  avail  myself  of  its  assistance.  I  can  only  now  recom- 
mend it  as  an  indispensable  part  of  the  outfit  of  every 
serious  student  of  the  petrographical  section  of  Geology. 

In  the  revision  of  the  stratigraphical  portion  of  this 
work  I  have  been  assisted  with  suggestions  and  informa- 
tion by  my  colleagues,  Mr.  Topley,  Mr.  H.  B.  Woodward, 
Mr.  E.  T.  Newton,  and  Mr.  C.  Reid,  to  whom  my  best 
thanks  are  due. 

MUSEUM,  JERMYX  STREET, 
1st  August  1893. 


FROM  THE  PREFACE  TO  THE  FIRST  EDITION 

THE  method  of  treatment  adopted  in  this  Text-book 
is  one  which,  while  conducting  the  class  of  Geology  in 
the  University  of  Edinburgh,  I  have  found  to  afford  the 
student  a  good  grasp  of  the  general  principles  of  the  science, 
and  at  the  same  time  a  familiarity  with  and  interest  in  de- 
tails of  which  he  is  enabled  to  see  the  bearing  in  the  general 
system  of  knowledge.  A  portion  of  the  volume  appeared 
in  the  autumn  of  1879  as  the  article  " Geology"  in  the 
Encyclopaedia  Britannica.  My  leisure  since  that  date  has 
been  chiefly  devoted  to  expanding  those  sections  of 
the  treatise  which  could  not  be  adequately  developed  in 
the  pages  of  a  general  work  of  reference. 

While  the  book  will  not,  I  hope,  repel  the  general 
reader  who  cares  to  know  somewhat  in  detail  the  facts 
and  principles  of  one  of  the  most  fascinating  branches  of 
natural  history,  it  is  intended  primarily  for  students,  and 
is  therefore  adapted  specially  for  their  use.  The  digest 
given  of  each  subject  will  be  found  to  be  accompanied  by 
references  to  memoirs  where  a  fuller  statement  may  be 
sought.  It  has  long  been  a  charge  against  the  geologists 
of  Great  Britain  that,  like  their  countrymen  in  general, 
they  are  apt  to  be  somewhat  insular  in  their  conceptions, 
even  in  regard  to  their  own  branch  of  science.  Of  course, 
specialists  who  have  devoted  themselves  to  the  investiga- 
tion of  certain  geological  formations  or  of  a  certain  group 
of  fossil  animals,  have  made  themselves  familiar  with 
(6) 


FROM  THE  PREFACE  TO   THE  FIRST  EDITION  7 

what  has  been  written  upon  their  subject  in  other  coun- 
tries. But  I  am  afraid  there  is  still  not  a  little  truth  in 
the  charge,  that  the  general  body  of  geologists  here  is  but 
vaguely  acquainted  with  geological  types  and  illustrations 
other  than  such  as  have  been  drawn  from  the  area  of  the 
British  Isles.  More  particularly  is  the  accusation  true  in 
regard  to  American  geology.  Comparatively  few  of  us  have 
any  adequate  conception  of  the  simplicity  and  grandeur  of 
the  examples  by  which  the  principles  of  the  science  have 
been  enforced  on  the  other  side  of  the  Atlantic. 

Fully  sensible  of  this  natural  tendency,  I  have  tried  to 
keep  it  in  constant  view  as  a  danger  to  be  avoided  as  far 
as  the  conditions  of  my  task  would  allow.  ID  a  text-book 
designed  for  use  in  Britain,  the  illustrations  must  obviously 
be  in  the  first  place  British.  A  truth  can  be  enforced  much 
more  vividly  by  an  example  culled  from  familiar  ground 
than  by  one  taken  from  a  distance.  But  I  have  striven  to 
widen  the  vision  of  the  student  by  indicating  to  him  that 
while  the  general  principles  of  the  science  remain  uniform, 
they  receive  sometimes  a  clearer,  sometimes  a  somewhat 
different,  light  from  the  rocks  of  other  countries  than  our 
own.  If  from  these  references  he  is  induced  to  turn  to  the 
labors  of  our  fellow  workers  on  the  Continent,  and  to  share 
my  respect  and  admiration  for  them,  a  large  part  of  my 
design  will  have  been  accomplished.  If,  further,  he  is  led 
to  study  with  interest  the  work  of  our  brethren  across1  the 
Atlantic,  and  to  join  in  my  hearty  regard  for  it  and  for 
them,  another  important  section  of  my  task  will  have  been 
fulfilled.  And  if  in  perusing  these  pages  he  should  find 
in  them  any  stimulus  to  explore  nature  for  himself,  to 
wander  with  the  enthusiasm  of  a  true  geologist  over  the 
length  and  breadth  of  his  own  country,  and,  where  oppor- 


8  FROM  THE  PREFACE  TO  THE  FIRST  EDITION 

tunity  offers,  to  extend  his  experience  and  widen  his  sym- 
pathies by  exploring  the  rocks  of  other  lands,  the  remaining 
and  chief  part  of  my  aim  would  be  attained. 

The  illustrations  of  Fossils  in  Book  VI.  have  been 
chiefly  drawn  by  Mr.  George  Sharman;  a  few  by  Mr.  B. 
N.  Peach,  and  one  or  two  by  Dr.  B.  H.  Traquair,  F.B.S., 
to  all  of  whom  my  best  thanks  are  due.  The  publishers 
having  become  possessed  of  the  wood-blocks  of  Sir  Henry 
de  la  Beche's  "Geological  Observer,"  I  gladly  made  use 
of  them  as  far  as  they  could  be  employed  in  Books  III. 
and  IV.  Sir  Henry's  sketches  were  always  both  clear  and 
artistic,  and  I  hope  that  students  will  not  be  sorry  to  see 
some  of  them  revived.  They  are  indicated  by  the  letter 
(£).  The  engravings  of  the  microscopic  structure  of  rocks 
are  from  my  own  drawings,  and  I  have  also  availed  myself 
of  materials  from  my  sketch-books.  The  frontispiece  is 
a  reduction  of  a  drawing  by  Mr.  W.  H.  Holmes,  whose 
pictures  of  the  scenery  in  the  Far  West  of  the  United  States 
are  by  far  the  most  remarkable  examples  yet  attained  of 
the  union  of  artistic  effectiveness  with  almost  diagram- 
matic geological  distinctness  and  accuracy.  Captain  Button, 
of  the  Geological  Survey  of  the  United  States,  furnished  me 
with  this  drawing,  and  also  requested  Mr.  Holmes  to  make 
for  me  the  cation-sections  given  in  Book  VII.  To  both  of 
these  kind  friends  I  desire  to  acknowledge  my  indebtedness. 


CONTENTS 


INTRODUCTION  ...........        13 

BOOK   I 
COSMICAL  ASPECTS  OF  GEOLOGY,  21 

I.  RELATIONS  OF  THE  EARTH  IN  THE  SOLAR  SYSTEM          ...      23 
II.  FORM  AND  SIZE  OP  THE  EARTH      .......      31 

III.  MOVEMENTS  OF  THE  EARTH  IN  THEIR  GEOLOGICAL  RELATIONS       .      34 

1.  Rotation,  34—2.  Revolution,  36—3.  Precession  of  the  Equinoxes, 
37-4.  Change  in  the  obliquity  of  the  Ecliptic,  38—5.  Stability 
of  the  Earth's  Axis,  38—6.  Changes  of  the  Earth's  Centre  of 
Gravity,  43—7.  Results  of  the  Attractive  Influence  of  Sun  and 
Moon  on  the  Geological  Condition  of  the  Earth,  46—8.  Climate 
in  its  Geological  Relations,  49. 

BOOK  II 

GEOGNOSY:    AN  INVESTIGATION  OF  THE  MATERIALS  OF 
THE  EARTH'S  SUBSTANCE 

PART    I.—  A    GENERAL    DESCRIPTION    OF   THE    PARTS   OF 
THE  EARTH 

I.  THB  ENVELOPES—  ATMOSPHERE  AND  HYDROSPHERE         ...      62 
1.  The  Atmosphere,  63  —  2.  The  Oceans,  66. 

II.  THB  SOLID  GLOBE  OR  LITHOSPHERE         ......       74 

1.  The  Outer  Surface,  74—2.  The  Crust,  87—3.  The  Interior  or 
Nucleus,  89;  Evidence  of  Internal  Heat,  92;  Irregularities  in 
the  downward  Increment  of  Heat,  95;  Probable  Condition 
of  the  Earth's  Interior,  99—4.  Age  of  the  Earth  and  Measures 
of  Geological  Time,  107. 


(9) 


10  CONTENTS 


PART  II.— AN  ACCOUNT   OF   THE   COMPOSITION   OP  THE   EARTH'S 
CRUST— MINERALS   AND   ROCKS 

L  GENERAL  CHEMICAL  CONSTITUTION  OF  THE  CRUST         .       .        .112 
II.  ROOK-FORMING  MINERALS 119 

III.  DETERMINATION  OF  BOOKS 146 

i.  Megascopic  Examination,  146— ii.  Chemical  Analysis,  157— iii. 
Chemical  Synthesis,  ICO-iv.  Microscopic  Investigation,  101. 

IV.  GENERAL  OUTWARD  OR  MEGASCOPIC  CHARACTERS  OF  ROCKS        .     172 

1.  Structure,  ITS— 2.  Composition,  186-8.  State  of  Aggregation, 
187—4.  Color  and  Lustre,  189-6.  Feel  and  Smell,  101-6. 
Specific  Gravity,  198—7.  Magnetism,  19(2 

V.  MICROSCOPIC  CHARACTERS  OF  ROCKS 192 

1.  Microscopic  Elements  of  Bocks,  194—8.  Microscopic  Struc- 
tures of  Rocks,  808. 

VL  CLASSIFICATION  OF  ROCKS 218 

VII.  A  DESCRIPTION  OF  THE  MORE  IMPORTANT  ROCKS  OF  THE  EARTH'S 

CRUST 223 

i.  SEDIMENTARY 223 

A.  Fragmented,  (Clastic) 223 

1.  Gravel  and  Sand  Rocks  (Psamrnites) 224 

2.  Clay  Rocks  (Pelites) 233 

3.  Volcanic  Fragmented  Rocks  (Tuffs) 238 

4.  Fragmental  Rocks  of  Organic  Origin 243 

(1)  Calcareous,  243-(2)  Siliceous,  247— (8)  Phosphatic,  848— (4)  Car- 
bonaceous, 249— (5)  Ferruginous,  265. 

B.  Crystalline,     including     Rocks    formed    from     Chemical 

Precipitation 258 

ii.  MASSIVE,  ERUPTIVE,  IGNEOUS 269 

i.  Acid  Series,  272— ii.  Intermediate  Series,  284— iii.  Basic  Series,  298. 

iii.  SCHISTOSE  (METAMOEPHIC) 303 

1.  Argillites,  809—2.  Quartz-Bocks,  310-3.  Pyroxene-Bocks,  818-4. 
Hornblende-Bocks,  314-5.  Garnet-Bocks,  815-3.  Epidote-Bocks, 
315-7.  Chlorite-Bocks,  315-8.  Talc-Bocks,  815—9.  Olivine- 
Bocks,  or  Peridotites,  316-10.  Pelsitoid-Bocks,  816—11.  Quartz- 
and  Tourmaline-Bocks,  817—12.  Quartz-  and  Mica-Bocks,  317— 
18.  Quartz-  and  Felspar-Bocks,  819—14.  Quartz-,  Felspar-,  and 
Mica-Bocks,  320—15.  Quartz-,  Felspar-,  and  Garnet-Bocks, 
322-16.  Felspar-  and  Mica-Bocks,  822— Composition  of  some 
Schistose  Bocks,  323. 


CONTENTS  11 

BOOK  III 

DYNAMICAL  GEOLOGY,  324 

PART  L— HYPOGENE   ACTION:    AN   INQUIRY  INTO  THE  GEOLOGI- 
CAL   CHANGES    IN    PROGRESS    BENEATH    THE    SURFACE 
OP   THE   EARTH,  326 

L  VOLCANOES  AND  VOLCANIC  ACTION 

1.  Volcanic  Products 327 

1.  Gases  and  Vapors,  380—2.  Water,  386-8.  Lava,  888—4.  Frag- 
mentary Materials,  340. 

2.  Volcanic  Action 344 

Active,  Dormant,  and  Extinct  Phases,  344— Sites  of  Volcanic 
Action,  346— Ordinary  Phase  of  an  Active  Volcano,  349— 
Conditions  of  Eruption,  849— Periodicity  of  Eruptions,  852— 
General  Sequence  of  Events  in  an  Eruption,  854 — Fissures, 
355— Explosions,  360— Showers  of  Dust  and  Stones,  868— Lava- 
streams,  370— Elevation  and  Subsidence,  895— Torrents  of 
Water  and  Mud,  396— Effects  of  the  closing  of  a  Volcanic 
Chimney— Sills  and  Dikes,  398— Exhalations  of  Vapors  and 
Gases,  899-Geysers,  402— Mud- Volcanoes,  407. 

3.  Structure  of  Volcanoes 409 

i.  Volcanic  Cones,  409— Submarine  Volcanoes,  and  Volcanic 
Islands,  434— ii.  Fissure  (Massive)  Eruptions,  432. 

4.  Geographical  and  Geological  Distribution  of  Volcanoes         .         .     439 

5.  Causes  of  Volcanic  Action 447 

II.  EARTHQUAKES  .  459 

Amplitude  of  Earth-Movements,  461-Velocity,  461-Duration,  468 
—Modifying  Influence  of  Geological  Structure,  463— Extent  of 
Country  affected,  466— Depth  of  Source,  466-Geological  Effects, 
468-Distribution,  478— Origin,  474. 

III.  SECULAR  UPHEAVAL  AND  DEPRESSION 478 

Upheaval,  482—  Subsidence,  489— Causes  of  Upheaval  and  Depres- 
sion of  Land,  494. 


TEXT-BOOK   OF  GEOLOGY 


INTRODUCTION 

GEOLOGY  is  the  science  which  investigates  the  history 
of  the  Earth.  Its  object  is  to  trace  the  progress  of  our 
planet  from  the  earliest  beginnings  of  its  separate  existence, 
through  its  various  stages  of  growth,  down  to  the  present 
condition  of  things.  It  unravels  the  complicated  processes, 
involving  vast  geographical  revolutions,  by  which  each 
continent  and  country  has  been  built  up,  tracing  out  the 
origin  of  their  materials  and  the  manner  in  which  their 
existing  outlines  have  been  determined.  It  likewise  fol- 
lows into  detail  the  varied  sculpture  of  mountain  and 
valley,  crag  and  ravine. 

Nor  does  this  science  confine  itself  merely  to  changes 
in  the  inorganic  world.  Geology  shows  that  the  present 
races  of  plants  and  animals  are  the  descendants  of  other 
and  very  different  races  that  once  peopled  the  earth.  It 
teaches  that  there  has  been  a  progress  of  the  inhabitants, 
as  well  as  one  of  the  globe  on  which  they  have  dwelt;  that 
each  successive  period  in  the  earth's  history,  since  the  intro- 
duction of  living  things,  has  been  marked  by  characteristic 
types  of  the  animal  and  vegetable  kingdoms;  and  that,  how 
imperfectly  soever  they  may  have  been  preserved  or  may 
be  deciphered,  materials  exist  for  a  history  of  life  upon  the 

(13) 


14  TEXT-BOOK   OF   GEOLOGY 

planet.  The  geographical  distribution  of  existing  faunas 
and  floras  is  often  made  clear  and  intelligible  by  geological 
evidence;  and  in  a  similar  way,  light  is  thrown  upon  some 
of  the  remoter  phases  in  the  history  of  man  himself. 

A  subject  so  comprehensive  as  this  must  require  a  wide 
and  varied  basis  of  evidence.  One  of  the  characteristics  of 
geology  is  to  gather  evidence  from  sources  which,  at  first 
sight,  seem  far  removed  from  its  scope,  and  to  seek  aid 
from  almost  every  other  leading  branch  of  science.  Thus, 
in  dealing  with  the  earliest  conditions  of  the  planet,  the 
geologist  must  fully  avail  himself  of  the  labors  of  the  as- 
tronomer. Whatever  is  ascertainable  by  telescope,  spec- 
troscope, or  chemical  analysis,  regarding  the  constitution 
of  other  heavenly  bodies,  has  a  geological  bearing.  The 
experiments  of  the  physicist,  undertaken  to  determine  con- 
ditions of  matter  and  of  energy,  may  sometimes  be  taken 
as  the  starting-point  of  geological  investigation.  The  work 
of  the  chemical  laboratory  forms  the  foundation  of  a  vast 
and  increasing  mass  of  geological  inquiry.  To  the  botanist, 
the  zoologist,  even  to  the  unscientific,  if  observant,  traveller 
by  land  or  sea,  the  geologist  turns  for  information  and 
assistance. 

But  while  thus  culling  freely  from  the  dominions  of 
other  sciences,  geology  claims,  as  its  peculiar  territory,  the 
rocky  framework  of  the  globe.  In  the  materials  composing 
that  framework,  their  composition  and  arrangement,  the 
processes  of  their  formation,  the  changes  which  they  have 
individually  undergone,  and  the  grand  terrestrial  revolu- 
tions to  which  they  bear  witness,  lie  the  main  data  of 
geological  history.  It  is  the  task  of  the  geologist  to  group 
these  elements  in  such  a  way  that  they  may  be  made  to 
yield  up  their  evidence  as  to  the  march  of  events  in  the 


INTRODUCTION  15 

evolution  of  the  planet.  He  finds  that  they  have  in  large 
measure  arranged  themselves  in  chronological  sequence — 
the  oldest  lying  at  the  bottom  and  the  newest  at  the  top. 
Eelics  of  an  ancient  sea -floor  are  overlain  with  traces  of 
a  vanished  land-surface;  these  are  in  turn  covered  by  the 
deposits  of  a  former  lake,  above  which  once  more  appear 
proofs  of  the  return  of  the  sea.  Among  these  rocky  records, 
too,  lie  the  lavas  and  ashes  of  long-extinct  volcanoes.  The 
ripple  left  upon  a  sandy  beach,  the  cracks  formed  by  the 
sun's  heat  upon  the  muddy  bottom  of  a  dried-up  pool, 
the  very  imprint  of  the  drops  of  a  passing  rain-shower, 
have  all  been  accurately  preserved,  and  often  bear  witness 
to  geographical  conditions  widely  different  from  those  that 
exist  where  such  markings  are  now  found. 

But  it  is  mainly  by  the  remains  of  plants  and  animals 
imbedded  in  the  rocks  that  the  geologist  is  guided  in  un- 
ravelling the  chronological  succession  of  geological  changes. 
He  has  found  that  a  certain  order  of  appearance  charac- 
terizes these  organic  remains;  that  each  successive  group 
of  rocks  is  marked  by  its  own  special  types  of  life;  that 
these  types  can  be  recognized,  and  the  rocks  in  which  they 
occur  can  be  correlated,  even  in  distant  countries,  where  no 
other  means  of  comparison  are  available.  At  one  moment, 
he  has  to  deal  with  the  bones  of  some  large  mammal  scat- 
tered through  a  deposit  of  superficial  gravel,  at  another 
time,  with  the  minute  foraminifers  and  ostracods  of  an 
upraised  sea-bottom.  Corals  and  crinoids,  crowded  and 
crushed  into  a  massive  limestone  on  the  spot  where  they 
lived  and  died,  ferns  and  terrestrial  plants  matted  together 
into  a  bed  of  coal  where  they  originally  grew,  the  scattered 
shells  of  a  submarine  sand-bank,  the  snails  and  lizards  that 
left  their  mouldering  remains  within  a  hollow  tree,  the  in- 


16  TEXT-BOOK   OF   GEOLOGY 

sects  that  have  been  imprisoned  within  the  exuding  resin 
of  old  forests,  the  footprints  of  birds  and  quadrupeds,  or 
the  trails  of  worms  left  upon  former  shores — these,  and 
innumerable  other  pieces  of  evidence,  enable  the  geologist 
to  realize  in  some  measure  what  the  vegetable  and  animal 
life  of  successive  periods  has  been,  and  what  geographical 
changes  the  site  of  every  land  has  undergone. 

It  is  evident  that  to  deal  successfully  with  these 
varied  materials,  a  considerable  acquaintance  with  differ- 
ent branches  of  science  is  desirable.  The  fuller  and  more 
accurate  the  knowledge  which  the  geologist  has  of  kindred 
branches  of  inquiry,  the  more  interesting  and  fruitful  will 
be  his  own  researches.  From  its  very  nature,  geology 
demands  on  the  part  of  its  votaries  wide  sympathy  with 
investigation  in  almost  every  branch  of  natural  science. 
Especially  necessary  is  a  tolerably  large  acquaintance  with 
the  processes  now  at  work  in  changing  the  surface  of  the 
earth,  and  of  at  least  those  forms  of  plant  and  animal  life 
whose  remains  are  apt  to  be  preserved  in  geological  deposits, 
or  which,  in  their  structure  and  habitat,  enable  us  to  realize 
what  their  forerunners  were. 

It  has  often  been  insisted  upon  that  the  Present  is  the  key 
to  the  Past;  and  in  a  wide  sense  this  assertion  is  eminently 
true.  Only  in  proportion  as  we  understand  the  present, 
where  everything  is  open  on  all  sides  to  the  fullest  inves- 
tigation, can  we  expect  to  decipher  the  past,  where  so  much 
is  obscure,  imperfectly  preserved,  or  not  preserved  at  all. 
A  study  of  the  existing  economy  of  nature  ought  evidently 
to  be  the  foundation  of  the  geologist's  training. 

While,  however,  the  present  condition  of  things  is  thus 
employed,  we  must  obviously  be  on  our  guard  against  the 
danger  of  unconsciously  assuming  that  the  phase  of  nature's 


INTRODUCTION  17 

operations  which  we  now  witness  has  been  the  same  in 
all  past  time;  that  geological  changes  have  taken  place, 
in  former  ages,  in  the  manner  and  on  the  scale  which  we 
behold  to-day,  and  that  at  the  present  time  all  the  great 
geological  processes,  which  have  produced  changes  in  past 
eras  of  the  earth's  history,  are  still  existent  and  active. 
Of  course,  we  may  assume  this  uniformity  of  action,  and 
use  the  assumption  as  a  working  hypothesis.  But  it  ought 
not  to  be  allowed  a  firmer  footing,  nor  on  any  account  be 
suffered  to  blind  us  to  the  obvious  truth  that  the  few  cen- 
turies, wherein  man  has  been  observing  nature,  form  much 
too  brief  an  interval  by  which  to  measure  the  intensity  of 
geological  action  in  all  past  time.  For  aught  we  can  tell, 
the  present  is  an  era  of  quietude  and  slow  change,  compared 
with  some  of  the  eras  that  have  preceded  it.  Nor  can  we 
be  sure  that  when  we  have  explored  every  geological  proc- 
ess now  in  progress,  we  have  exhausted  all  the  causes  of 
change  which,  even  in  comparatively  recent  times,  have 
been  at  work. 

In  dealing  with  the  Geological  Record,  as  the  accessible 
solid  part  of  the  globe  is  called,  we  cannot  too  vividlj 
realize  that,  at  the  best,  it  forms  but  an  imperfect  chroni- 
cle. Geological  history  cannot  be  compiled  from  a  full  and 
continuous  series  of  documents.  Owing  to  the  very  nature 
of  its  origin,  the  record  is  necessarily  from  the  first  frag- 
mentary, and  it  has  been  farther  mutilated  and  obscured  by 
the  revolutions  of  successive  ages.  Even  where  the  chroni- 
cle of  events  is  continuous,  it  is  of  very  unequal  value  in 
different  places.  In  one  case,  for  example,  it  may  present 
us  with  an  unbroken  succession  of  deposits,  many  thou- 
sands of  feet  in  thickness,  from  which,  however,  only  a  few 
meagre  facts  as  to  geological  history  can  be  gleaned.  In 


18  TEXT-BOOK   OF   GEOLOGY 

another  instance,  it  brings  before  us,  within  the  compass 
of  a  few  yards,  the  evidence  of  a  most  varied  and  com- 
plicated series  of  changes  in  physical  geography,  as  well 
as  an  abundant  and  interesting  suite  of  organic  remains. 
These  and  other  characteristics  of  the  geological  record 
will  become  more  apparent  and  intelligible  to  the  student 
as  he  proceeds  in  the  study  of  the  science. 

In  the  present  volumes  the  subject  will  be  distributed 
under  the  following  leading  divisions: 

1.  The   Gosmical  Aspects  of  Geology. — It  is  desirable  to 
realize  some  of  the  more  important  relations  of  the  earth 
to  the  other  members  of  the  solar  system,  of  which  it  forms 
a  part,   seeing  that   geological  phenomena  are   largely  the 
result  of   these   relations.     The  form   and  motions   of   the 
planet  may  be  briefly  touched  upon,  and  attention  should 
be  directed  to  the  way  in  which  these  planetary  movements 
influence  geological  change.     The  light  cast  upon  the  early 
history  of  the  earth  by  researches  into  the  composition  of 
the  sun  and  stars  deserves  notice  here. 

2.  Geognosy — An  Inquiry  into  the  Materials  of  the  Earth's 
Substance. — This  division  describes  the  constituent  parts  of 
the  earth,  its  envelopes  of  air  and  water,  its  solid  crust, 
and  the  probable  condition  of  its  interior.     Especially,  it 
directs    attention    to    the   more   important  minerals   of   the 
crust,  and  the  chief  rocks  of  which  that  crust  is  built  up. 
In  this  way,  it  lays  a  foundation  of  knowledge  regarding 
the  nature  of   the  materials   constituting    the  mass  of  the 
globe,  whence  we  may  next  proceed  to  investigate  the  proc- 
esses by  which  these  materials  are  produced  and  altered. 

8.  Dynamical  Geology  embraces  an  investigation  of  the 
operations  which  lead  to  the  formation,  alteration,  and  dis- 
turbance of  rocks,  and  calls  in  the  aid  of  physical  and 


INTRODUCTION  19 

chemical  experiment  in  elucidation  of  these  operations. 
It  considers  the  nature  and  operation  of  the  processes  that 
have  determined  the  distribution  of  sea  and  land,  and  have 
molded  the  forms  of  the  terrestrial  ridges  and  depressions. 
It  further  investigates  the  geological  changes  which  are  in 
progress  over  the  surface  of  the  land  and  floor  of  the  sea, 
whether  these  are  due  to  subterranean  disturbance,  or  to 
the  effect  of  operations  above  ground.  Such  an  inquiry 
necessitates  a  careful  study  of  the  existing  economy  of 
nature,  and  forms  a  fitting  introduction  to  the  investiga- 
tion of  the  geological  changes  of  former  periods.  This 
and  the  previous  section,  including  most  of  what  is 
embraced  under  Physical  Geography  and  Petrogeny  or 
Geogeny,  will  here  be  discussed  more  in  detail  than  is 
nsual  in  geological  treatises. 

4.  Geotectonic,  or  Structural  Geology — the  Architecture  of 
the  Earth. — This  section  of  the  investigation,  applying  the 
results  arrived  at  in  the  previous  division,  discusses  the 
actual  arrangement  of  the  various  materials  composing 
the  crust  of  the  earth.  It  proves  that  some  have  been 
formed  in  beds  or  strata,  whether  by  the  deposit  of  sedi- 
ment on  the  floor  of  the  sea,  or  by  the  slow  aggregation  of 
organic  forms,  that  others  have  been  poured  out  from  sub- 
terranean sources  in  sheets  of  molten  rock,  or  in  showers  of 
loose  dust,  which  have  been  built  up  into  mountains  and 
plateaus.  It  further  shows  that  rocks  originally  laid  down 
in  almost  horizontal  beds  have  subsequently  been  crum- 
pled, contorted,  dislocated,  invaded  by  igneous  masses  from 
below,  and  rendered  sometimes  crystalline.  It  teaches, 
too,  that  wherever  exposed  above  sea-level,  they  have  been 
incessantly  worn  down,  and  have  often  been  depressed,  so 
that  older  lie  buried  beneath  later  accumulations. 


20  TEXT-BOOK   OF   GEOLOGY 

5.  Palceontological  Geology. — This  branch  of  the  subject 
deals  with  the  organic  forms  which  are  found  preserved  in 
the  rocks  of  the  crust  of  the  earth.     It  includes  such  ques- 
tions as  the   manner  in   which  the  remains  of  plants  and 
animals  are   entombed    in   sedimentary  accumulations,   the 
relations  between  extinct  and  living  types,  the  laws  which 
appear  to   have  governed  the  distribution  of  life  in  time 
and  in  space,   the  nature   and   use  of   the  evidence  from 
organic   remains   regarding   former  conditions   of    physical 
geography,  and  the  relative  importance  of  different  genera 
of  animals  and  plants  in  geological  inquiry. 

6.  Stratigraphical  Geology. — This  section  might  be  called 
Geological  History,   or  Historical  Geology.     It  works  out 
the  chronological  succession  of  the  great  formations  of  the 
earth's  crust,  and  endeavors  to  trace  the  sequence  of  events 
of  which    they  contain  the  record.     More  particularly,   it 
determines  the  order  of  succession   of   the  various   plants 
and  animals  which  in  past  time  have  peopled  the  earth, 
and  thus,  by  ascertaining  what  has  been  the  grand  march 
of  life  upon  the  planet,  seeks  to  unravel  the  story  of  the 
earth  as  made  known  by  the  rocks  of  the  crust.     Further, 
by  comparing  the  sequence  of  rocks  in  one  country  with 
that  of  those  in  another,  it  furnishes  materials  for  enabling 
us  to  picture  the  successive  stages  in  the  geographical  evo- 
lution of  the  various  portions  of  the  earth's  surface. 

7.  Physiographical   Geology,   starting  from   the  basis  of 
fact  laid  down  by  Stratigraphical  geology  regarding  former 
geographical  changes,  embraces  an  inquiry  into  the  history 
of  the  present  features  of  the  earth's  surface — continental 
ridges  and  ocean  basins,  plains,  valleys,  and  mountains.     It 
investigates  the  structure  of  mountains  and  valleys,  com- 
pares the  mountains  of  different  countries,  and  ascertains 


COSMIC AL   ASPECTS    OF   GEOLOGY  21 

the  relative  geological  dates  of  their  upheaval.  It  explains 
the  causes  on  which  local  differences  of  scenery  depend, 
and  shows  under  what  very  different  circumstances,  and 
at  what  widely  separated  intervals,  the  varied  contours, 
even  of  a  single  country,  have  been  produced. 


BOOK  I 

COSMICAL  ASPECTS  OF  GEOLOGY 

BEFOEE  geology  had  attained  to  the  position  of  an 
inductive  science,  it  was  customary  to  begin  all 
investigations  into  the  history  of  the  earth  by  pro- 
pounding or  adopting  some  more  or  less  fanciful  hypoth- 
esis, in  explanation  of  the  origin  of  our  planet  or  of  the 
universe.  Such  preliminary  notions  were  looked  upon  as 
essential  to  a  right  understanding  of  the  manner  in  which 
the  materials  of  the  globe  had  been  put  together.  To  the 
illustrious  James  Hutton  (1785)  geologists  are  indebted,  if 
not  for  originating,  at  least  for  strenuously  upholding  the 
doctrine  that  it  is  no  part  of  the  province  of  geology 
to  discuss  the  origin  of  things.  He  taught  them  that  in 
the  materials  from  which  geological  evidence  is  to  be  com- 
piled there  can  be  found  "no  traces  of  a  beginning,  no 
prospect  of  an  end."  In  England,  mainly  to  the  influence 
of  the  school  which  he  founded,  and  to  the  subsequent  rise 
of  the  Geological  Society  (1807),  which  resolved  to  collect 
facts  instead  of  fighting  over  hypotheses,  is  due  the  dis- 
appearance of  the  crude  and  unscientific  cosmologies  of 
previous  centuries. 

But  there  can  now  be  little  doubt  that  in  the  reaction 


22  TEXT-BOOK    OF   GEOLOGY 

against  the  visionary  and  often  grotesque  speculations  of 
earlier  writers,  geologists  were  carried  too  far  in  an  oppo- 
site direction.  In  allowing  themselves  to  believe  that 
geology  had  nothing  to  do  with  questions  of  cosmogony, 
they  gradually  grew  up  in  the  conviction  that  such  ques- 
tions could  never  be  other  than  mere  speculation,  interest- 
ing or  amusing  as  a  theme  for  the  employment  of  the  fancy, 
but  hardly  coming  within  the  domain  of  sober  and  inductive 
science.  Nor  would  they  soon  have  been  awakened  out  of 
this  belief  by  anything  in  their  own  science.  It  is  still  true 
that  in  the  data  with  which  they  are  accustomed  to  deal, 
as  comprising  the  sum  of  geological  evidence,  there  can 
be  found  no  trace  of  a  beginning,  though  there  is  ample 
proof  of  constant,  upward  progression  from  some  invisible 
starting-point.  The  oldest  rocks  which  have  been  discov- 
ered on  any  part  of  the  globe  have  possibly  been  derived 
from  other  rocks  older  than  themselves.  Geology  by  itself 
has  not  yet  revealed,  and  is  little  likely  ever  to  reveal, 
a  portion  of  the  first  solid  crust  of  our  globe.  If,  then, 
geological  history  is  to  be  compiled  from  direct  evidence 
furnished  by  the  rocks  of  the  earth,  it  cannot  begin  at  the 
beginning  of  things,  but  must  be  content  to  date  its  first 
chapter  from  the  earliest  period  of  which  any  record  has 
been  preserved  among  the  rocks. 

Nevertheless,  though,  in  its  usual  restricted  sense,  geol- 
ogy has  been,  and  must  ever  be,  unable  to  reveal  the 
earliest  history  of  our  planet,  it  no  longer  ignores,  as  mere 
speculation,  what  is  attempted  in  this  subject  by  its  sister 
sciences.  Astronomy,  physics  and  chemistry  have  in  late 
years  all  contributed  to  cast  much  light  on  the  earliest 
stages  of  the  earth's  existence,  previous  to  the  beginning 
of  what  is  commonly  regarded  as  geological  history.  What- 


COSMIC AL    ASPECTS   OF   GEOLOGY  23 

ever  extends  oar  knowledge  of  the  former  conditions  of  our 
globe  may  be  legitimately  claimed  as  part  of  the  domain 
of  geological  inquiry.  If  Geology,  therefore,  is  to  continue 
worthy  of  its  name  as  the  science  of  the  earth,  it  must  take 
cognizance  of  these  recent  contributions  from  other  sciences. 
It  can  no  longer  be  content  to  begin  its  annals  with  the 
records  of  the  oldest  rocks,  but  must  endeavor  to  grope  its 
way  through  the  ages  which  preceded  the  formation  of  any 
rocks.  Thanks  to  the  results  achieved  with  the  telescope, 
the  spectroscope,  and  the  chemical  laboratory,  the  story 
of  these  earliest  ages  of  our  earth  is  every  year  becoming 
more  definite  and  intelligible. 

I.  BELATIOXS  OF  THE  EARTH  IN  THE  SOLAR  SYSTEM 

As  a  prelude  to  the  study  of  the  structure  and  history 
of  the  earth,  some  of  the  general  relations  of  our  planet  to 
the  solar  system  may  here  be  noticed.  The  investigations 
of  recent  years,  showing  the  community  of  substance  be- 
tween the  different  members  of  that  system,  have  revived 
and  have  given  a  new  form  and  meaning  to  the  well-known 
nebular  hypothesis  of  Kant,  Laplace  and  W.  Herschel, 
which  sketched  the  progress  of  the  system  from  the  state 
of  an  original  nebula  to  its  existing  condition  of  a  central 
incandescent  sun  with  surrounding  cool  planetary  bodies. 
According  to  this  hypothesis,  the  nebula,  originally  diffused 
at  least  as  far  as  the  furthest  member  of  the  system,  began 
to  condense  toward  the  centre,  and  in  so  doing  threw  off 
or  left  behind  successive  rings.  These,  on  disruption  and 
further  condensation,  assumed  the  form  of  planets,  some- 
times with  a  further  formation  of  rings,  which  in  the  case 
of  Saturn  remain,  though  in  other  planets  they  have  broken 
up  and  united  into  satellites. 


24  TEXT-BOOK   OF   GEOLOGY 

Accepting  this  view,  we  should  expect  the  matter  com* 
posing  the  various  members  of  the  solar  system  to  be  every- 
where  nearly  the  same.  The  fact  of  condensation  round 
centres,  however,  indicates  probable  differences  of  density 
throughout  the  nebula.  That  the  materials  composing  the 
nebula  may  have  arranged  themselves  according  to  their 
respective  densities,  the  lightest  occupying  the  exterior, 
and  the  heaviest  the  interior  of  the  mass,  is  suggested  by 
a  comparison  of  the  densities  of  the  various  planets.  These 
densities  are  usually  estimated  as  in  the  following  table, 
that  of  the  earth  being  taken  as  the  unit: 

Density  of  the  Sun 0-25 

Mercury 1-12 

Venus..- 1-03 

Earth 1-00 

Mars 0-70 

Jupiter 0-24 

Saturn 0'13 

Uranus 0-17 

Neptune. 0'16 

It  is  to  be  observed,  however,  that  "the  densities  here 
given  are  mean  densities,  assuming  that  the  apparent  size 
of  the  planet  or  sun  is  the  true  size,  i.e.  making  no  allow- 
ance for  thousands  of  miles  deep  of  cloudy  atmosphere. 
Hence  the  numbers  for  Jupiter,  Saturn  and  Uranus  are 
certainly  too  small,  that  for  the  sun  much  too  small."1 
Taking  the  figures  as  they  stand,  while  they  do  not  indi- 
cate a  strict  progression  in  the  diminution  of  density,  they 
state  that  the  planets  near  the  sun  possess  a  density  about 
twice  as  great  as  that  of  granite,  but  that  those  lying  toward 
the  outer  limits  of  the  system  are  composed  of  matter  as 
light  as  cork.  Again,  in  some  cases,  a  similar  relation  has 
been  observed  between  the  densities  of  the  satellites  and 

1  Prof.  Tait,  MS.  note. 


COSMIC 'AL  ASPECTS  OF  GEOLOGY         ^ 

their  primaries.  The  moon,  for  example,  has  a  density 
little  more  than  half  that  of  the  earth.  The  first  satellite 
of  Jupiter  is  less  dense,  though  the  other  three  are  said  to 
be  more  dense  than  the  planet.  Further,  in  the  condition 
of  the  earth  itself,  a  very  light  gaseous  atmosphere  forms 
the  outer  portion,  beneath  which  lies  a  heavier  layer  of 
water,  while  within  these  two  envelopes  the  materials  form- 
ing the  solid  substance  of  the  planet  are  so  arranged  that 
the  outer  layer  or  crust  has  only  about  half  the  density  of 
the  whole  globe. 

According  to  the  hypothesis  now  under  consideration 
it  is  conceived  that,  in  the  gradual  condensation  of  the 
original  nebula,  each  successive  mass  left  behind  repre- 
sented the  density  of  its  parent  shell,  and  consisted  of 
progressively  heavier  matter."  The  remoter  planets,  with 
their  low  densities  and  vast  absorbing  atmospheres,  may 
be  supposed  to  consist  of  metalloids,  like  the  outer  parts 
of  the  sun's  atmosphere,  while  the  interior  planets  are  no 
doubt  mainly  metallic.  The  rupture  of  each  planetary  ring 
would,  it  is  thought,  raise  the  temperature  of  the  resultant 
nebulous  planet  to  such  a  height  as  to  allow  the  vapors 
to  rearrange  themselves  by  degrees  in  successive  layers,  or 
rather  shells,  according  to  densities.  And  when  the  planet 
gave  off  a  satellite,  that  body  might  be  expected  to  possess 
the  composition  and  density  of  the  outer  layers  of  its 
primary.8 

For  many  years,  the  only  evidence  available  as  to  the 

2  On  the  origin  of  Satellites,  see  the  researches  of  Prof.  G.  H.  Darwin,  Phil. 
Trans.  (1879)  clxx.  p.  535.     Proc.  Roy.  Soc.  xxx.  p.  1. 

3  Lockyer  in  Prestwich's  Inaugural  Lecture,   Oxford,   1875,  and  in  Man- 
chester Lectures,  Why  the  Earth's  Chemistry  is  as  it  is.     Readers  interested 
in  the  historical  development  of  geological  opinion  will  find  much  suggestive 
matter  bearing  on  the  questions  discussed  above,  in  De  la  Beche's  "Researches 
in  Theoretical  Geology,"  1834— a  work  notably  in  advance  of  its  time. 

GEOLOGY— Vol.  XXIX— 2 


26  TEXT-BOOK   OF   GEOLOGY 

actual  composition  of  other  heavenly  bodies  than  our  own 
earth  was  furnished  by  the  meteorites,  or  fallen  stars,  which 
from  time  to  time  have  entered  our  atmosphere  from  plan- 
etary space,  and  have  descended  upon  the  surface  of  the 
globe.4  Subjected  to  chemical  analysis,  these  foreign 
bodies  show  considerable  diversities  of  composition;  but 
in  no  case  have  they  yet  revealed  the  existence  of  any 
element  not  already  recognized  among  terrestrial  materials. 
They  have  been  classified  in  three  groups:  Siderites,  com- 
posed chiefly  of  iron;  Siderolites,  consisting  partly  of  iron 
and  partly  of  various  stony  materials ;  and  Aerolites,  formed 
almost  entirely  of  such  stony  minerals.  Twenty-four  of  our 
elements  have  been  detected  in  meteorites.  Those  most 
commonly  found  are  iron,  nickel,  phosphorus,  sulphur, 
carbon,  oxygen,  silicon,  magnesium,  calcium  and  alumin- 
ium. Less  frequent  or  occurring  in  smaller  quantities  are 
hydrogen,  nitrogen,  chlorine,  lithium,  sodium,  potassium, 
titanium,  chromium,  manganese,  cobalt,  arsenic,  antimony, 
tin  and  copper.  These  various  elements  occur  for  the  most 
part  in  a  state  of  combination.  The  iron  exists  as  an  alloy 
with  nickel,  the  phosphorus  is  combined  with  nickel  and 
iron,  the  silicon  is  combined  with  oxygen  and  various  bases. 
A  few  of  the  elements  occur  in  a  free  state.  Thus  hydrogen 
and  nitrogen  are  found  as  occluded  gases  and  carbon  as 


4  On  meteorites  consult  Partsch,  "Die  Meteoriten,"  Vienna,  1843.  Rose, 
Abhand.  konigl.  Akad.  Berlin,  1863.  Rammelsberg,  "Die  Chemische  Natur 
der  Meteoriten,"  1870-9.  Tschermak,  Sitzb.  Akad.  "Wissen.  Vienna  (1875), 
Ixxi. ;  "Die  Mikroskopische  Beschaffenheit  der  Meteoriten,"  Stuttgart,  1885. 
Daubree,  "Etudes  Synthetiques  de  Geologie  Experimentale, "  1879;  Compt. 
rend.  cvi.  (1888),  1671-1682  (compare  Amer.  Journ.  Sci.  xlii.  [1891],  p.  413). 
S.  Meunier,  "Le  Ciel  Geologique,"  1871;  "Meteorites,"  1884.  Brezina  tmd 
Coheu,  "Die  Structur  und  Zusammensetzung  der  Meteoreisen,"  Stuttgart,  1886. 
W.  Flight,  Geol.  Mag.  1875,  Pop.  Sci.  Rev.  new  ser.  i.  p.  390.  Proe.  Roy.  Soc. 
xxxiii.  p.  343.  A.  W.  Wright,  Amer.  Journ.  ser.  3,  xi.  p.  253;  xii.  p.  165. 
L.  Fletcher,  "An  Introduction  to  the  Study  of  Meteorites,"  British  Museum 
Catalogue,  1886. 


COSMIC AL   ASPECTS    OF   GEOLOGY  27 

graphite,  rarely  as  diamond.  Of  combinations  of  elements 
in  meteorites  some,  not  yet  recognized  among  terrestrial 
minerals,  comprise  alloys  of  iron  and  nickel  and  various 
sulphides  and  silicates.  But  others  have  been  identified 
with  well-known  minerals  of  the  earth's  crust,  including 
olivine,  enstatite  and  bronzite,  diopside  and  augite,  horn- 
blende, anorthite  and  labradorite,  magnetite  and  chromite, 
etc.  There  is  likewise  a  carbonaceous  group  of  meteorites 
containing  carbon,  both  amorphous  and  as  black  diamond, 
also  combined  with  hydrogen  and  oxygen,  and  in  some 
cases  combustible,  with  a  bituminous  smell.  Some  iron 
meteorites  contain  a  large  proportion  of  occluded  hydro- 
gen, nitrogen,  or  carbonic  oxide,  occasionally  as  much  as 
six  times  the  volume  of  the  meteorite  itself. 

Various  theories  have  been  propounded  as  to  the  origin 
or  source  of  those  bodies  which  come  to  our  planet  from 
space.  But  at  present  we  possess  no  satisfactory  basis  of 
fact  on  which  to  speculate.  Whether  these  stones  belong 
to  the  solar  system,  or,  as  seems  more  probable,  reach  us 
from  remoter  space,  they  prove  that  some  at  least  of  the 
elements  and  minerals  with  which  we  are  familiar  extend 
beyond  our  planet. 

But,  in  recent  years,  a  far  more  precise  and  generally 
available  method  of  research  into  the  composition  of  the 
heavenly  bodies  has  been  found  in  the  application  of  the 
spectroscope.  By  means  of  this  instrument,  the  light 
emitted  from  self-luminous  bodies  can  be  analyzed  in 
such  a  way  as  to  show  what  elements  are  present  in  their 
intensely  hot  luminous  vapor.  When  the  light  of  the  in- 
candescent vapor  of  a  metal  is  allowed  to  pass  through 
a  properly  arranged  prism,  it  is  seen  to  give  a  spectrum 
consisting  of  transverse  bright  lines  only.  This  is  termed 


28  TEXT-BOOK   OF   GEOLOGY 

a  radiation -spectrum.  Each  element  appears  to  have  its 
own  characteristic  arrangement  of  lines,  which  in  general 
retain  the  same  relative  position,  intensity  and  colors. 
Moreover,  gases  and  the  vapors  of  solid  bodies  are  found 
to  intercept  those  rays  of  light  which  they  themselves 
emit.  The  spectrum  of  sodium-vapor,  for  example,  shows 
among  others  two  bright  orange  lines.  If  therefore  white 
light,  from  some  hotter  light-source,  passes  through  the 
vapor  of  sodium,  these  two  bright  lines  become  dark  lines, 
the  light  being  exactly  cut  off  which  would  have  been 
given  out  by  the  sodium  itself.  This  is  called  an  absorption- 
spectrum. 

From  this  method  of  examination,  it  has  been  inferred 
that  many  of  the  elements  of  which  our  earth  is  composed 
must  exist  in  the  state  of  incandescent  vapor  in  the  atmos- 
phere of  the  sun.  Thirty-two  metals  have  been  thus  iden- 
tified, including  aluminium,  barium,  manganese,  lead,  cal- 
cium, cobalt,  potassium,  iron,  zinc,  copper,  nickel,  sodium 
and  magnesium.  These  elements,  or  at  least  substances 
which  give  the  same  groups  of  lines  as  the  terrestrial  ele- 
ments with  which  they  have  been  identified,  do  not  occur 
promiscuously  diffused  throughout  the  outer  mass  of  the 
sun.  According  to  Mr.  Lockyer's  first  observations,  they 
appear  to  succeed  each  other  in  relation  to  their  respective 
densities.  Thus  the  coronal  atmosphere  which,  as  seen  in 
total  eclipses,  extends  to  so  prodigious  a  distance  beyond 
the  disk  of  the  sun,  consists  mainly  of  subincandescent 
hydrogen  and  another  element  which  may  be  new.  Beneath 
this  external  vaporous  envelope  lies  the  chromosphere, 
where  the  vapors  of  incandescent  hydrogen,  calcium  and 
magnesium  can  be  detected.  Further  inward  the  spot-zone 
shows  the  presence  of  sodium,  titanium,  etc. ;  while  still 


COSM1CAL  ASPECTS  OF  GEOLOGY        29 

lower,  a  layer  (the  reversing  layer)  of  intensely  hot  vapors, 
lying  probably  next  to  the  inner  brilliant  photosphere,  gives 
spectroscopic  evidence  of  the  existence  of  incandescent  iron, 
manganese,  cobalt,  nickel,  copper,  and  other  well-known 
terrestrial  metals.6 

It  is  to  be  observed,  however,  that  in  these  spectroscopic 
researches  the  decomposition  of  the  elements  by  electrical 
action  was  not  considered.  The  conclusions  embodied  in 
the  foregoing  paragraph  have  been  founded  on  the  idea  that 
the  lines  seen  in  the  spectrum  of  any  element  are  all  due  to 
the  vibrations  of  the  molecules  of  that  element.  But  Mr. 
Lockyer  has  suggested  that  this  view  may  after  all  be  but  a 
rough  approximation  to  the  truth;  that  it  may  be  more  ac- 
curate to  say,  as  a  result  of  the  facts  already  acquired,  that 
there  exist  basic  elements  common  to  calcium,  iron,  etc., 
and  to  the  solar  atmosphere,  and  that  the  spectrum  of  each 
body  is  a  summation  of  the  spectra  of  various  molecular 
complexities  which  can  exist  at  different  temperatures,  the 
simplest  only  being  found  in  the  hottest  part  of  the  sun.8 

The  spectroscope  has  likewise  been  successfully  applied 
by  Mr.  Huggins  and  others  to  the  observation  of  the  fixed 
stars  and  nebulas,  with  the  result  of  establishing  a  similarity 
of  elements  between  our  own  system  and  other  bodies  in 

6  On  spectroscopic  research  as  applied  to  the  sun,  see  Kirchhoff  and  Bunsen, 
"Researches  on  Solar  Spectrum,"  etc.,  1863;  Angstrom,  "Eecherches  sur  le 
Spectre  normal  du  Soleil";  Lockyer,  "Solar  Physics,"  1873,  and  "StudieS  in 
Spectrum  Analysis"  (International  Series),  1878;  Huggins  and  Miller,  Proc. 
Roy.  Soc.  xii.,  Phil.  Trans.  1864;  Roscoe's  "Spectrum  Analysis,"  with  au- 
thorities there  cited.  An  ingenious  theory  to  account  for  the  conservation  of 
solar  energy  was  suggested  by  the  late  Sir  C.  W.  Siemens  (Proc.  Roy.  Soc. 
xxxiii.  (1881)  p.  389).  It  requires  the  presence  of  aqueous  vapor  and  carbon 
compounds  in  stellar  space,  which  are  dissociated  and  drawn  into  the  solar 
photosphere,  where  they  burst  into  flame  with  a  large  development  of  heat,  and 
then  passing  into  aqueous  vapor  and  carbonic  anhydride  or  oxide,  flow  to  the 
solar  equator  whence  they  are  projected  into  space. 

8  See  also  the  opposite  views  of  Dewar  and  Liveing,  Proc.  Roy.  Soc.  xxx. 
p.  93,  and  H.  W.  Vogel,  Nature,  xxvii.  p.  233. 


80  TEXT-BOOK   OF   GEOLOGY 

sidereal  space.  In  the  radiation-spectra  of  nebulae,  Mr. 
Huggins  finds  the  hydrogen  lines  very  prominent;  and  he 
conceives  that  they  may  be  glowing  masses  of  that  element. 
Prof.  Tait  has  suggested,  on  the  other  hand,  that  they  are 
more  probably  clouds  of  stones  frequently  colliding  and 
thus  giving  off  incandescent  gases.  Sir  William  Thomson 
(now  Lord  Kelvin)  favors  this  view,  which  is  further  amply 
supported  by  spectroscopic  observations.  Among  the  fixed 
stars,  absorption-spectra  have  been  recognized,  pointing  to 
a  structure  resembling  that  of  our  sun,  viz.  an  incandescent 
nucleus  which  may  be  solid  or  liquid  or  of  very  highly  com- 
pressed gas,  but  which  gives  a  continuous  spectrum  and 
which  is  surrounded  with  an  atmosphere  of  glowing  vapor.7 
Those  stars  which  show  the  simplest  spectra  are  believed  to 
have  the  highest  temperature,  and  in  proportion  as  they  cool 
their  materials  will  become  more  and  more  differentiated 
into  what  we  call  elements.  The  most  brilliant  or  hottest 
stars  show  in  their  spectra  only  the  lines  of  gases,  as  hydro- 
gen. Cooler  stars,  like  our  sun,  give  indications  of  the 
presence,  in  addition,  of  the  metals — magnesium,  sodium, 
calcium,  iron.  A  still  lower  temperature  is  marked  by  the 
appearance  of  the  other  metals,  metalloids,  and  compounds.8 
The  sun  would  thus  be  a  star  considerably  advanced  in  the 
process  of  differentiation  or  association  of  its  atoms.  It  con- 
tains, so  far  as  we  know,  no  metalloid  except  carbon,  and 
possibly  oxygen,  nor  any  compound,  while  stars  like  Sirius 
show  the  presence  only  of  hydrogen,  with  but  a  feeble  pro- 
portion of  metallic  vapors;  and,  on  the  other  hand,  the  red 
stars  indicate  by  their  spectra  that  their  metallic  vapors  have 


7  Huggins,  Proc.  Roy.  Soc.  1863-66,  and  Brit.  Aaaoc.  Lecture  (Nottingham, 
1866);  Huggins  and  Miller,  Phil.  Trans.  1864. 

8  Lockyer,  Comptes  rendus,  Dec.  1873. 


COSMIC AL  ASPECTS  OF  GEOLOGY         31 

entered  into  combination,  whence  it  is  inferred  that  their 
temperature  is  lower  than  that  of  our  sun. 

More  recently,  however,  another  view  of  the  evolution 
of  stars  has  been  propounded  by  Mr.  Lockyer.  He  con- 
ceives that  all  self-luminous  cosmical  bodies  are  composed 
either  of  swarms  of  meteorites,  or  of  masses  of  vapor  pro- 
duced by  collisions  of  meteorites:  that  stars,  comets  and 
nebulas  are  only  different  phases  of  the  same  series  of 
changes;  that  where  the  temperature  of  a  star  is  increasing 
the  star  consists  of  a  meteor-swarm,  which  by  constant  col- 
lision of  its  individual  meteorites  is  gradually  being  vapor- 
ized by  heat;  and  that  after  volatilization  cooling  sets  in  and 
the  vapor  finally  condenses  into  a  globe.9 

II.  FORM  AND  SIZE  OF  THE  EARTH 

Further  confirmation  of  some  of  the  foregoing  views  as 
to  the  order  of  planetary  evolution  is  furnished  by  the  form 
of  the  earth  and  the  arrangement  of  its  component  materials. 

That  the  earth  is  an  oblate  spheroid,  and  not  a  perfectly 
spherical  globe,  was  discovered  and  demonstrated  by  New- 
ton. He  even  calculated  the  amount  of  ellipticity  long  be- 
fore any  measurement  had  confirmed  such  a  conclusion. 
During  the  present  century  numerous  arcs  of  the  meridian 
have  been  measured,  chiefly  in  the  northern  hemisphere. 
From  a  series  made  by  different  observers  between  the  l#ti- 
tudes  of  Sweden  and  the  Cape  of  Good  Hope,  Bessel  ob- 
tained the  following  data  for  the  dimensions  of  the  earth: 

Equatorial  diameter     .     .     .     41,847,192  feet,  or  7925-604  miles 

Polar  diameter 41,707,314       "       7899-114     " 

Amount  of  polar  flattening    .          139,768       "  26-471     " 

»  "The  Meteoritic  Hypothesis,"  1890. 


32  TEXT-BOOK    OF   GEOLOGY 

The  equatorial  circumference  is  thus  a  little  less  than 
25,000  miles,  and  the  difference  between  the  polar  and  equa- 
torial diameters  (nearly  26*  miles)  amounts  to  about  *ioth  of 
the  equatorial  diameter.10  More  recently,  however,  it  haa 
been  shown  that  the  oblate  spheroid  indicated  by  these  meas- 
urements is  not  a  symmetrical  body,  the  equatorial  circum- 
ference being  an  ellipse  instead  of  a  circle.  The  greater 
axis  of  the  equator  lies  in  long.  8°  15'  W. — a  meridian  pass- 
ing through  Ireland,  Portugal,  and  the  northwest  corner  of 
Africa,  and  cutting  off  the  northeast  corner  of  Asia  in  the 
opposite  hemisphere." 

The  polar  flattening,  established  by  measurement  and 
calculation  as  that  which  would  necessarily  have  been  as- 
sumed by  an  originally  plastic  globe  in  obedience  to  the 
movement  of  rotation,  has  been  cited  as  evidence  that  the 
earth  was  once  in  a  plastic  condition.  Taken  in  connection 
with  the  analogies  supplied  by  the  sun  and  other  heavenly 
bodies,  this  inference  appeared  to  be  well  grounded."  More 
recently,  however,  it  has  been  contended  that  even  in  a  truly 
solid  body  a  polar  flattening  might  be  developed  under  the 
influence  of  rotation." 

Though  the  general  spheroidal  form  of  our  planet,  and 

19  Herechel,  "Astronomy,"  p.  186. 

11  A.  R.  Clarke,  Phil.  Mag.  August,  1878;  Encyclopedia  Britannica,  9th 
edit.  x.  172. 

18  It  was  opposed  by  Mohr  ("Gescbichte  der  Erde,"  p,  472),  who,  adopting 
a  suggestion  long  ago  made  by  Playfair,  endeavored  to  show  that  the  polar  flat- 
tening can  be  accounted  for  by  greater  denudation  of  the  polar  tracts,  exposed 
as  these  have  been  by  the  heaping  up  of  the  oceanic  waters  toward  the  equator 
in  consequence  of  rotation.  He  dwelt  chiefly  on  the  effects  of  glaciers  in  lower- 
ing the  land,  but  as  Pfaff  has  pointed  out,  the  work  of  erosion  is  chiefly  per- 
formed by  other  atmospheric  forces  that  operate  rather  toward  the  equator  than 
the  poles  ("Allgemeine  Geologie  als  exacte  Wissenschaft, "  p.  6).  Compare 
Naumann,  Neues  Jahrb.  1871,  p.  250.  Nevertheless,  Mohr  undoubtedly  re- 
called attention  to  a  conceivable  cause  by  which,  in  spite  of  polar  elevation  or 
equatorial  subsidence,  the  external  form  of  the  planet  might  be  preserved. 

18  See  in  particular  the  papers  by  Mr.  C.  Chree.  Phil.  Mag.  1891,  pp.  238 
and  342. 


COSMIC AL   ASPECTS   OF   GEOLOGY  33 

probably  the  general  distribution  of  sea  and  land,  are  refer- 
able to  the  early  effects  of  rotation  on  a  fluid  or  viscous 
mass,  it  is  certain  that  the  present  details  of  its  surface-con- 
tours are  of  comparatively  recent  date.  Speculations  have 
been  made  as  to  what  may  have  been  the  earliest  character 
of  the  solid  surface,  whether  it  was  smooth  or  rough,  and 
particularly  whether  it  was  marked  by  any  indication  of  the 
existing  continental  elevations  and  oceanic  depressions.  So 
far  as  we  can  reason  from  geological  evidence,  there  is  no 
proof  of  any  uniform  superficies  having  ever  existed.  Most 
probably  the  first  formed  crust  broke  up  irregularly,  and 
not  until  after  many  successive  corrugations  did  the  surface 
acquire  stability.  Some  writers  have  imagined  that  at  first 
the  ocean  spread  over  the  whole  surface  of  the  planet.  But 
of  this  there  is  not  only  no  evidence,  but  good  reason  for  be- 
lieving that  it  never  could  have  taken  place.  As  will  be 
alluded  to  in  a  later  page,  the  preponderance  of  water  in  the 
southern  hemisphere  seems  to  indicate  some  excess  of  den- 
sity in  that  hemisphere.  This  excess  can  hardly  have  been 
produced  by  any  change  since  the  materials  of  the  interior 
ceased  to  be  mobile;  it  must  therefore  be  at  least  as  ancient 
as  the  condensation  of  water  on  the  earth's  surface.  Hence 
there  was  probably  from  the  beginning  a  tendency  in  the 
ocean  to  accumulate  in  the  southern  rather  than  in  the 
northern  hemisphere. 

That  land  existed  from  the  earliest  ages  of  which  'we 
have  any  record  in  rock-formations,  is  evident  from  the 
obvious  fact  that  these  formations  themselves  consist  in 
great  measure  of  materials  derived  from  the  waste  of  land. 
When  the  student,  in  a  later  part  of  these  volumes,  is  pre- 
sented with  the  proofs  of  the  existence  of  enormous  masses 
of  sedimentary  deposits,  even  among  some  of  the  oldest  geo- 


34  TEXT-BOOK  OF   GEOLOGY 

logical  systems,  he  will  perceive  how  important  mast  have 
been  the  tracts  of  land  that  could  furnish  such  piles  of 
detritus. 

The  tendency  of  modern  research  is  to  give  probability 
to  the  conception,  first  outlined  by  Kant,  that  not  only  in 
our  own  solar  system,  but  throughout  the  regions  of  space, 
there  has  been  a  common  plan  of  evolution,  and  that  the 
matter  diffused  through  space  in  nebulae,  stars  and  planets 
is  substantially  the  same  as  that  with  which  we  are  familiar. 
Hence  the  study  of  the  structure  and  probable  history  of  the 
sun  and  the  other  heavenly  bodies  comes  to  possess  an  evi- 
dent geological  interest,  seeing  that  it  may  yet  enable  us  to 
carry  back  the  story  of  our  planet  far  beyond  the  domain  of 
ordinary  geological  evidence,  and  upon  data  not  less  trust- 
worthy than  those  furnished  by  the  rocks  of  the  earth's 
crust. 

III.  MOVEMENTS  OF  THE  EARTH  IN  THEIR  GEOLOGICAL 
•  KELATIONS 

We  are  here  concerned  with  the  earth's  motions  in  so  far 
only  as  they  materially  influence  the  progress  of  geological 
phenomena. 

§  1.  Rotation. — In  consequence  of  its  angular  momentum 
at  its  original  separation,  the  earth  rotates  on  its  axis.  The 
rate  of  rotation  has  once  been  much  more  rapid  than  it  now 
is  (p.  46).  At  present  a  complete  rotation  is  performed  in 
about  twenty-four  hours,  and  to  it  is  due  the  succession 
of  day  and  night.  So  far  as  observation  has  yet  gone,  this 
movement  is  uniform,  though  recent  calculations  of  the  in- 
fluence of  the  tides  in  retarding  rotation  tend  to  show  that 
a  very  slow  diminution  of  the  angular  velocity  is  in  prog- 
ress. If  this  be  so,  the  length  of  the  day  and  night  will 


COSMICAL  ASPECTS  OF  GEOLOGY        35 

slowly  increase  until  finally  the  duration  of  the  day  and 
that  of  the  year  will  be  equal.  The  earth  will  then  have 
reached  the  condition  into  which  the  moon  has  passed  rela- 
tively to  the  earth,  one-half  being  in  continual  day,  the 
other  in  perpetual  night. 

The  linear  velocity  due  to  rotation  varies  in  different 
places,  according  to  their  position  on  the  surface  of  the 
planet.  At  each  pole  there  can  be  no  velocity,  but  from 
these  two  points  toward  the  equator  there  is  a  continually 
increasing  rapidity  of  motion,  till  at  the  equator  it  is  equal 
to  a  rate  of  507  yards  in  a  second. 

To  the  rotation  of  the  earth  are  due  certain  remarkable 
influences  upon  currents  of  air  circulating  either  toward  the 
equator  or  toward  the  poles.  Currents  which  move  from 
polar  latitudes  travel  from  parts  of  the  earth's  surface 
where  the  velocity  due  to  rotation  is  small,  to  others  where 
it  is  great.  Hence  they  lag  behind,  and  their  course  is 
bent  more  and  more  westward.  An  air  current,  quitting 
the  north  polar  or  north  temperate  regions  as  a  north  wind, 
is  deflected  out  of  its  course,  and  becomes  a  northeast  wind. 
On  the  opposite  side  of  the  equator,  a  similar  current  set- 
ting out  straight  for  the  equator,  is  changed  into  a  south- 
east wind.  Hence,  as  is  well  known,  the  Trade-winds  have 
their  characteristic  westward  deflection.  On  the  other 
hand,  a  current  setting  out  northward  or  southward  from 
the  equator,  passes  into  regions  having  a  less  velocity'  due 
to  rotation  than  it  possesses  itself,  and  hence  it  travels  on 
in  advance  and  appears  to  be  gradually  deflected  eastward. 
The  aerial  currents,  blowing  steadily  across  the  surface  of 
the  ocean  toward  the  equator,  produce  oceanic  currents 
which  unite  to  form  the  westward  flowing  Equatorial 
current. 


36  TEXT-BOOK   OF   GEOLOGY 

It  has  been  maintained  by  Von  Baer,14  that  a  certain 
deflection  is  experienced  by  rivers  that  flow  in  a  meridi- 
onal direction,  like  the  Volga  and  Irtisch.  Those  travel- 
ling poleward  are  asserted  to  press  upon  their  eastern  rather 
than  their  western  banks,  while  those  which  run  in  the  op- 
posite direction  are  stated  to  be  thrown  more  against  the 
western  than  the  eastern.  When,  however,  we  consider 
the  comparatively  small  volume,  slow  motion,  and  contin- 
ually meandering  course  of  rivers,  it  may  reasonably  be 
doubted  whether  this  vera  causa  can  have  had  much  effect 
generally  in  modifying  the  form  of  river  channels. 

§  2.  Revolution. — Besides  turning  on  its  axis,  the  globe 
performs  a  movement  round  the  sun,  termed  revolution. 
This  movement,  accomplished  in  rather  more  than  365 
days,  determines  for  us  the  length  of  our  year,  which  is, 
in  fact,  merely  the  time  required  for  one  complete  revolu- 
tion. The  path  or  orbit  followed  by  the  earth  round  the 
sun  is  not  a  perfect  circle  but  an  ellipse,  with  the  sun 
in  one  of  the  foci,  the  mean  distance  of  the  earth  from 
the  sun  being  92,800,000,  the  present  aphelion  distance 
94,500,000,  and  the  perihelion  distance  91,250,000  miles. 
By  slow  secular  variations,  the  form  of  the  orbit  alter- 
nately approaches  to  and  recedes  from  that  of  a  circle. 
At  the  nearest  possible  approach  between  the  two  bodies, 


14  "Ueber  ein  allgemeines  Gesetz  in  der  Gestaltung  der  Flussbetten. "  Bull 
Acad.  St.  Petersbourg,  ii.  (1860).  See  also  Ferrel  on  the  motion  of  fluids  and 
solids  relatively  to  the  earth's  surface,  Camb.  (Mass.)  Math.  Monthly,  vols.  i. 
and  ii.  (1859-60);  Bulk,  Z.  Deutsch.  Geol.  Ges.  xxxi.  (1879)  p.  224.  The  River 
Irtisch  is  said  in  flowing  northward  to  have  cut  so  much  into  its  right  bank  that 
villages  are  gradually  driven  eastward,  Demiansk  having  been  shifted  about  a 
mile  in  240  years  (Nature,  xv.  p.  207).  But  this  may  be  accounted  for  by  local 
causes.  See  an  excellent  paper  on  this  subject  with  special  reference  to  the 
regime  of  some  rivers  in  northern  Germany,  by  F.  Klockmann,  Jahrb.  Preuss. 
Geol.  Landesanst.  1882;  also  E.  Bunker,  Zeitsch.  fur  die  gesammten  Nattirwis- 
senschaften,  1876,  p.  463;  G.  K.  Gilbert,  Amer.  Journ.  Sci.  xxvii.  (1884)  p.  427. 


COSMIC AL   ASPECTS    OF   GEOLOGY  37 

owing  to  change  in  the  ellipticity  of  the  orbit,  the  earth  is 
14,368,200  miles  nearer  the  sun  than  when  at  its  greatest 
possible  distance.  These  maxima  and  minima  of  distance 
occur  at  vast  intervals  of  time."  The  last  considerable 
eccentricity  took  place  about  200,000  years  ago,  and  the 
previous  one  more  than  half  a  million  years  earlier.  Since 
the  amount  of  heat  received  by  the  earth  from  the  sun  is 
inversely  as  the  square  of  the  distance,  eccentricity  may 
have  had  in  past  time  much  effect  upon  the  climate  of  the 
earth,  as  will  be  pointed  out  further  on  (§  8). 

§  3.  Precession  of  the  Equinoxes.— If  the  axis  of  the  earth 
were  perpendicular  to  the  plane  of  its  orbit,  there  would  be 
equal  day  and  night  all  the  year  round.  But  it  is  really 
inclined  from  that  position  at  an  angle  of  23°  27'  21".  Hence 
our  hemisphere  is  alternately  presented  to  and  turned  away 
from  the  sun,  and,  in  this  way,  brings  the  familiar  alterna- 
tion of  the  seasons.  Again,  were  the  earth  a  perfect  sphere, 
of  uniform  density  throughout,  the  position  of  its  axis  of 
rotation  would  not  be  changed  by  attractions  of  external 
bodies.  But  owing  to  the  protuberance  along  the  equato- 
rial regions,  the  attraction  chiefly  of  the  moon  and  sun 
tends  to  pull  the  axis  aside,  or  to  make  it  describe  a 
conical  movement,  like  that  of  the  axis  of  a  top,  round 
the  vertical.  Hence  each  pole  points  successively  to  dif- 
ferent stars.  This  movement,  called  the  precession  of  the 
equinoxes,  in  combination  with  another  smaller  movement, 
due  to  the  attraction  of  the  moon  (called  nutation),  com- 
pletes its  cycle  in  21,000  years,  the  annual  total  advance 
of  the  equinox  amounting  to  62".  At  present  the  winter  in 
the  northern  hemisphere  coincides  with  the  earth's  nearest 

15  See  Croll'a  "Climate  and  Time,"  chaps,  iv.  xix. 


88  TEXT-BOOK   OF   GEOLOGY 

approach  to  the  sun,  or  perihelion.  In  10,500  years  hence 
it  will  take  place  when  the  earth  is  at  the  furthest  part  of 
its  orbit  from  the  sun,  or  in  aphelion.  This  movement 
may  have  had  great  importance  in  connection  with  former 
secular  variations  in  the  eccentricity  of  the  orbit  (§  8). 

§  4.  Change  in  the  Obliquity  of  the  Ecliptic.— The  angle 
at  which  the  axis  of  the  earth  is  inclined  to  the  plane 
of  its  orbit  does  not  remain  strictly  constant.  It  oscillates 
through  long  periods  of  time  to  the  extent  of  about  a  degree 
and  a  half,  or  perhaps  a  little  more,  on  either  side  of  the 
mean.  According  to  Dr.  Croll,1'  this  oscillation  must  have 
considerably  affected  former  conditions  of  climate  on  the 
earth,  since,  when  the  obliquity  is  at  its  maximum,  the  polar 
regions  receive  about  eight  and  a  half  days'  more  of  heat 
than  they  do  at  present — that  is,  about  as  much  heat  as  lat. 
76°  enjoys  at  this  day.  This  movement  must  have  aug- 
mented the  geological  effects  of  precession,  to  which  refer- 
ence has  just  been  made,  and  which  are  described  in  §  8. 

§  5.  Stability  of  the  Earth's  Axis.— That  the  axis  of  the 
earth's  rotation  has  successively  shifted,  and  consequently 
that  the  poles  have  wandered  to  different  points  on  the  sur- 
face of  the  globe,  has  been  maintained  by  geologists  as  the 
only  possible  explanation  of  certain  remarkable  conditions 
of  climate,  which  can  be  proved  to  have  formerly  obtained 
within  the  Arctic  Circle.  Even  as  far  north  as  lat.  81°  45', 
abundant  remains  of  a  vegetation  indicative  of  a  warm 
climate,  and  including  a  bed  of  coal  25  to  30  feet  thick, 
have  been  found  in  situ."  It  is  contended  that  when  these 
plants  lived,  the  ground  could  not  have  been  permanently 


16  Croll,  Trans.  Greol.  Soc.  Glasgow,  ii.  177.     "Climate  and  Time,"  chap.  xxv. 
11  Fielden  and  Heer,  Quart.  Journ.  Qeol.  Soc.  Nov.  1877. 


COSMIC AL    ASPECTS   OF   GEOLOGY  39 

frozen  or  covered  for  most  of  the  year  with  thick  snow.  In 
explanation  of  the  difficulty,  it  has  been  suggested  that  the 
north  pole  did  not  occupy  its  present  position,  and  that  the 
locality  where  the  plants  occur  lay  in  more  southerly  lati- 
tudes. Without  at  present  entering  on  the  discussion  of 
the  question  whether  the  geological  evidence  necessarily 
requires  so  important  a  geographical  change,  let  us  con- 
sider how  far  a  shifting  of  the  axis  of  rotation  has  been  a 
possible  cause  of  change  during  that  section  of  geological 
time  for  which  there  are  records  among  the  stratified  rocks. 
From  the  time  of  Laplace,"  astronomers  hare  strenu- 
ously denied  the  possibility  of  any  sensible  change  in  the 
position  of  the  axis  of  rotation.  It  has  been  urged  that, 
since  the  planet  acquired  its  present  oblate  spheroidal  form, 
nothing  but  an  utterly  incredible  amount  of  deformation 
could  overcome  the  greater  centrifugal  force  of  the  equato- 
rial protuberance.  It  is  certain,  however,  that  the  axis  of 
rotation  does  not  strictly  coincide  with  the  principal  axis 
of  inertia.  Though  the  angular  difference  between  them 
must  always  have  been  small,  we  can,  without  having  re- 
course to  any  extramundane  influence,  recognize  two  causes 
which,  whether  or  not  they  may  suffice  to  produce  any 
change  in  the  position  of  the  main  axis  of  inertia,  undoubt- 
edly tend  to  do  so.  In  the  first  place,  a  widespread  up- 
heaval or  depression  of  certain  unsymmetrically  arranged 
portions  of  the  surface  to  a  considerable  amount  would  tend 
to  shift  that  axis.  In  the  second  place,  an  analogous  result 
might  arise  from  the  denudation  of  continental  masses  of 
land,  and  the  consequent  filling  up  of  sea- basins.  Lord 
Kelvin  (Sir  William  Thomson)  freely  concedes  the  physical 

18  Mecanique  Celeste,"  tome  v.  p.  14. 


40  TEXT-BOOK    OF   GEOLOGY 

possibility  of  such  changes.  "We  may  not  merely  admit," 
he  says,  "but  assert  as  highly  probable,  that  the  axis  of 
maximum  inertia  and  axis  of  rotation,  always  very  near  one 
another,  may  have  been  in  ancient  times  very  far  from  their 
present  geographical  position,  and  may  have  gradually 
shifted  through  10,  20,  30,  40,  or  more  degrees,  without 
at  any  time  any  perceptible  sudden  disturbance  of  either 
land  or  water."19  But  though,  in  the  earlier  ages  of  the 
planet's  history,  stupendous  deformations  may  have  oc- 
curred, and  the  axis  of  rotation  may  have  often  shifted, 
it  is  only  the  alterations  which  can  possibly  have  occurred 
during  the  accumulation  of  the  stratified  rocks,  that  need 
to  be  taken  into  account  in  connection  with  the  evidence  of 
changes  of  climate  during  geological  history.  If  it  can  be 
shown,  therefore,  that  the  geographical  revolutions  neces- 
sary to  shift  the  axis  are  incredibly  stupendous  in  amount, 
improbable  in  their  distribution,  and  not  really  demanded 
by  geological  evidence,  we  may  reasonably  withhold  our 
belief  from  this  alleged  cause  of  the  changes  of  climate  dur- 
ing the  periods  of  time  embraced  by  geological  records. 

It  has  been  estimated  by  Lord  Kelvin  "that  an  elevation 
of  600  feet,  over  a  tract  of  the  earth's  surface  1000  miles 
square  and  10  miles  in  thickness,  would  only  alter  the 
position  of  the  principal  axis  by  one-third  of  a  second,  or 
84feet."**  Professor  George  Darwin  has  shown  that,  on 
the  supposition  of  the  earth's  complete  rigidity,  no  redis 
tribution  of  matter  in  new  continents  could  ever  shift  the 
pole  from  its  primitive  position  more  than  8°,  but  that,  if 
its  degree  of  rigidity  is  consistent  with  a  periodical  readjust- 


19  Brit.  Assoc.  Rep.  (1876),  Sections,  p.  11. 

20  Trans.  Geol.  Soc.  Glasgow,  iv.   p.   313.     The  situation  of  the  supposed 
area  of  upheaval  on  the  earth's  surface  is  not  stated. 


COSMIC AL   ASPECTS   OF   GEOLOGY  41 

rnent  to  a  new  form  of  equilibrium,  the  pole  may  have 
wandered  some  10°  or  15°  from  its  primitive  position,  or 
have  made  a  smaller  excursion  and  returned  to  near  its 
old  place.  In  order,  however,  that  these  maximum  effects 
should  be  produced,  it  would  be  necessary  that  each  ele- 
vated area  should  have  an  area  of  depression  corresponding 
in  size  and  diametrically  opposite  to  it,  that  they  should 
lie  on  the  same  complete  meridian,  and  that  they  should 
both  be  situated  in  lat.  45°.  With  all  these  coincident 
favorable  circumstances,  an  effective  elevation  of  aJo  of 
the  earth's  surface  to  the  extent  of  10,000  feet  would  shift 
the  pole  11 J';  a  similar  elevation  of  «  would  move  it  1°  46"; 
of  ,10,  3°  17';  and  of  J,  8°  4J'.  Mr.  Darwin  admits  these  to 
be  superior  limits  to  what  is  possible,  and  that,  on  the 
supposition  of  intumescence  or  contraction  under  the  re- 
gions in  question,  the  deflection  of  the  pole  might  be  reduced 
to  a  quite  insignificant  amount." 

Under  the  most  favorable  conditions,  therefore,  the  pos 
sible  amount  of  deviation  of  the  pole  from  its  first  position 
would  appear  to  have  been  too  small  to  have  seriously  in- 
fluenced the  climates  of  the  globe  within  geological  history. 
If  we  grant  that  these  changes  were  cumulative,  and  that 
the  superior  limit  of  deflection  was  reached  only  after  a 
long  series  of  concurrent  elevations  and  depressions,  we 
must  suppose  that  no  movements  took  place  elsewhere  to 
counteract  the  effect  of  those  about  lat.  45°  in  the  two  hemi- 
spheres. But  this  is  hardly  credible.  A  glance  at  a  geo- 
graphical globe  suffices  to  show  how  large  a  mass  of  land 
exists  now  both  to  the  north  and  south  of  that  latitude, 
especially  in  the  northern  hemisphere,  and  that  the  deepest 
parts  of  the  ocean  are  not  antipodal  to  the  greatest  heights 

81  Phil.  Trans.  Nov.  1876. 


42  TEXT-BOOK   OF   GEOLOGY 

of  the  land.  These  features  of  the  earth's  surface  are  of 
old  standing.  There  seems,  indeed,  to  be  no  geological 
evidence  in  favor  of  any  such  geographical  changes  as 
could  have  produced  even  the  comparatively  small  dis- 
placement of  the  axis  considered  possible  by  Professor 
Darwin. 

In  an  ingenious  suggestion,  Sir  John  Evans  contended 
that,  even  without  any  sensible  change  in  the  position  of 
the  axis  of  rotation  of  the  nucleus  of  the  globe,  there  might 
be  very  considerable  changes  of  latitude  due  to  disturbance 
of  the  equilibrium  of  the  outer  portion  or  shell  by  the  up- 
heaval or  removal  of  masses  of  land  between  the  equator 
and  the  poles,  and  to  the  consequent  sliding  of  the  shell 
over  the  nucleus  until  the  equilibrium  was  restored.22 
Subsequently  he  precisely  formulated  his  hypothesis  as 
a  question  to  be  determined  mathematically;"  and  the 
solution  of  the  problem  was  worked  out  by  the  Rev.  J.  F. 
Twisden,  who  arrived  at  the  conclusion  that  even  the  large 
amount  of  geographical  change  postulated  by  Dr.  Evans 
could  only  displace  the  earth's  axis  of  figure  to  the  extent 
of  less  than  10'  of  angle,  that  a  displacement  of  as  much  as 
10°  or  15°  could  be  effected  only  if  the  heights  and  depths 
of  the  areas  elevated  and  depressed  exceeded  by  many  times 
the  heights  of  the  highest  mountains,  that  under  no  circum- 
stances could  a  displacement  of  20°  be  effected  by  a  transfer 
of  matter  of  less  amount  than  about  a  sixth  part  of  the 
whole  equatorial  bulge,  and  that  even  this  extreme  amount 
would  not  necessarily  alter  the  position  of  the  axis  of 
figure." 


»  Proc.  Roy.  Soc.  xv.  (1867),  p.  46.      43  Q.  J.  Geol.  Soc.  xxxii.  (1876),  p.  62. 
24  Q.  J.  Geol.  Soc.  xxxiv.  (1878),  p.  41.     See  also  E.  Hill,  Geol.  Mag.  v. 
(2d  ser.)  pp.  262,  479.     0.  Fisher,  op.  cit.  pp.  291,  551. 


COSMIC 'AL    ASPECTS   OF   GEOLOGY  43 

Against  any  hypothesis  which  assumes  a  thin  crust  in- 
closing a  liquid  or  viscous  interior,  weighty  and,  indeed, 
insuperable  objections  have  been  urged.  It  has  been  sug- 
gested, however,  that  the  almost  universal  traces  of  present 
or  former  volcanic  action,  the  evidence  from  the  compressed 
strata  in  mountain  regions  that  the  crust  of  the  earth  must 
have  a  capacity  for  slipping  toward  certain  lines,  the  great 
amount  of  horizontal  compression  of  strata  which  can  be 
proved  to  have  been  accomplished,  and  the  secular  changes 
of  climate — notably  the  former  warm  climate  near  the  north 
pole — furnish  grounds  for  inquiry  whether  the  doctrine  of 
a  fluid  substratum  over  a  rigid  nucleus,  which  has  been 
urged  by  several  able  writers,  would  not  be  compatible 
with  mechanical  considerations,  and  whether,  under  these 
circumstances,  changes  in  latitude  would  not  result  from 
unequal  thickening  of  the  crust."  This  question  of  the 
internal  condition  of  the  globe  is  discussed  at  p.  89. 

§  6.  Changes  of  the  Earth's  Centre  of  Gravity.— If  the 
centre  of  gravity  in  our  planet,  as  pointed  out  by  Herschel, 
be  not  coincident  with  the  centre  of  figure,  but  lies  some- 
what to  the  south  of  it,  any  variation  in  its  position  will 
affect  the  ocean,  which  of  course  adjusts  itself  in  relation 
to  the  earth's  centre  of  gravity.  How  far  any  redistribu- 
tion of  the  matter  within  the  earth,  in  such  a  way  as  to 
affect  the  present  equilibrium,  is  now  possible,  we  cannot 
tell.  But  certain  revolutions  at  the  surface  may  from  time 
to  time  produce  changes  of  this  kind.  The  accumulation 
of  ice  which,  as  will  be  immediately  described  (§  8),  is 
believed  to  gather  round  one  pole  during  the  maximum 


»•  0.  Fisher,  Geol.  Mag.  1878,  p.  552,  "Physics  of  the  Earth's  Crust,' 
2d  Edition  1889. 


44  TEXT-BOOK    OF   GEOLOGY 

of  eccentricity,  will  displace  the  centre  of  gravity,  and, 
as  the  result  of  this  change,  will  raise  the  level  of  the 
ocean  in  the  glacial  hemisphere.1"  The  late  Dr.  Croll  esti 
mated  that,  if  the  present  mass  of  ice  in  the  southern  hemi- 
sphere is  taken  at  1000  feet  thick  extending  down  to  lat.  60°, 
the  transference  of  this  mass  to  the  northern  hemisphere 
would  raise  the  level  of  the  sea  80  feet  at  the  north  pole. 
Other  methods  of  calculation  give  different  results.  Mr. 
Heath  put  the  rise  at  128  feet;  Archdeacon  Pratt  made 
it  more;  while  the  Rev.  O.  Fisher  gave  it  at  409  feet." 
Subsequently,  in  returning  to  this  question,  Dr.  Croll  re- 
marked "that  the  removal  of  two  miles  of  ice  from  the 
Antarctic  continent  [and  at  present  the  mass  of  ice  there 
is  probably  thicker  than  that]  would  displace  the  centre  of 
gravity  190  feet,  and  the  formation  of  a  mass  of  ice  equal 
to  the  one-half  of  this,  on  the  Arctic  regions,  would  carry 
the  centre  of  gravity  95  feet  further;  giving  in  all  a  total 
displacement  of  285  feet,  thus  producing  a  rise  of  level  at 
the  north  pole  of  285  feet,  and  in  the  latitude  of  Edinburgh 
of  234  feet."  A  very  considerable  additional  displacement 
would  arise  from  the  increment  of  water  to  the  mass  of  the 
ocean  by  the  melting  of  the  ice.  Supposing  half  of  the  two 
miles  of  Antarctic  ice  to  be  replaced  by  an  ice-cap  of  similar 
extent  and  one  mile  thick  in  the  northern  hemisphere,  the 
other  half  being  melted  into  water  and  increasing  the  mass 
of  the  ocean,  Dr.  Croll  estimated  that  from  this  source  an 
extra  rise  of  200  feet  would  take  place  in  the  general  ocean 
level,  so  that  there  would  be  a  rise  of  485  feet  at  the  north 


26  Adhemar,  "Revolutions  de  la  Mer,"  1840. 

21  Croll,  in  Reader  for  2d  September,  1865,  and  Phil.  Mag.  April,  1866; 
Heath,  Phil.  Mag.  April,  1869;  Pratt,  Phil.  Mag.  March,  1866;  Fisher,  Reader, 
10th  February,  1866. 


COSMIC AL  ASPECTS  OF  GEOLOGY         45 

pole,  and  434  feet  in  the  latitude  of  Edinburgh."  An  inter- 
mittent submergence  and  emergence  of  the  low  polar  lands 
might  be  due  to  the  alternate  shifting  of  the  centre  of 
gravity. 

To  what  extent  this  cause  has  actually  come  into  opera- 
tion in  past  time  cannot  at  present  be  determined.  It  has 
been  suggested  that  the  "raised  beaches,"  shore-lines 
(strand-linien),  or  old  sea- terraces,  so  numerous  at  various 
heights  in  the  northwest  of  Europe,  might  be  due  to  the 
transference  of  the  oceanic  waters,  and  not  to  any  subter- 
ranean movement,  as  generally  believed.  Had  they  been 
due  to  such  a  general  cause,  they  ought  to  have  shown 
evidence  of  a  gradual  and  uniform  decline  in  elevation 
from  north  to  south,  with  only  such  local  variations  as 
might  be  accounted  for  by  the  influence  of  masses  of  high 
land  or  other  local  cause.  No  such  feature,  however,  has 
been  satisfactorily  established."  On  the  contrary,  the  levels 
of  the  terraces  vary  within  comparatively  short  distances. 
Though  numerous  on  both  sides  of  Scotland,  they  disap- 
pear further  north  among  the  Orkney  and  Shetland  islands, 
although  these  localities  were  admirably  adapted  for  their 
formation  and  preservation.80  The  conclusion  may  be 
drawn  that  the  "raised  beaches"  cannot  be  adduced  as 
evidence  of  changes  of  the  earth's  centre  of  gravity,  but 
are  due  to  local  and  irregularly  acting  causes.  (See  Book 
III.  Part  I.  Section  iii.  §  1,  where  this  subject  is  more 
fully  discussed.) 


28  Croll,  Geol.  Mag.  new  series,  i.  (1874),  p.  347;  "Climate  and  Time," 
chaps,  xxiii.  and  xxiv.  and  postea,  p.  286.  Consult  also  Fisher,  Phil.  Mag. 
xxxiv.  (October,  1892),  p.  337. 

**  The  student  ought,  however,  to  consult  Prof.  Suess'  Antlitz  der  Erde  for 
the  arguments  in  favor  of  an  opposite  opinion. 

30  Nature,  xvi.  (1877),  p.  415. 


46  TEXT-BOOK    OF   GEOLOGY 

§  7.  Results  of  the  Attractive  Influence  of  Sun  and  Moon 
on  the  Geological  Condition  of  the  Earth. — Many  speculations 
have  been  offered  to  account  for  supposed  former  greater 
intensity  of  geological  activity  on  the  surface  of  the  globe. 
Two  causes  for  such  greater  intensity  may  be  adduced.  In 
the  first  place,  if  the  earth  has  cooled  down  from  an  original 
molten  condition,  it  has  lost,  in  cooling,  a  vast  amount  of 
potential  geological  energy.  It  does  not  necessarily  follow, 
however,  that  the  geological  phenomena  resulting  from  in- 
ternal temperature  have,  during  the  time  recorded  in  the 
accessible  part  of  the  earth's  crust,  been  steadily  decreasing 
in  magnitude.  We  might,  on  the  contrary,  contend  that 
the  increased  resistance  of  a  thickening  cooled  crust  may 
rather  have  hitherto  intensified  the  manifestations  of  subter- 
ranean activity,  by  augmenting  the  resistance  to  be  over- 
come. In  the  second  place,  the  earth  may  have  been  once 
more  powerfully  affected  by  external  causes,  such  as  the 
greater  heat  of  the  sun,  and  the  greater  proximity  of  the 
moon.  That  the  formerly  larger  amount  of  solar  heat  re- 
ceived by  the  surface  of  our  planet  must  have  produced 
warmer  climates  and  more  rapid  evaporation,  with  greater 
rainfall  and  the  important  chain  of  geological  changes 
which  such  an  increase  would  introduce,  appears  in  every 
way  probable,  though  the  geologist  has  not  yet  been  able 
to  observe  any  indisputable  indication  of  such  a  former 
intensity  of  superficial  changes. 

Prof.  Darwin,  in  investigating  the  bodily  tides  of  viscous 
spheroids,  has  brought  forward  some  remarkable  results 
bearing  on  the  question  of  the  possibility  that  geological 
operations,  both  internal  and  superficial,  may  have  been 
once  greatly  more  gigantic  and  rapid  than  they  are  now.81 

31  Phil.  Trans.  1879,  parts  i.  and  ii. 


COSMIC AL   ASPECTS   OF   GEOLOGY  47 

He  assumes  the  earth  to  be  a  homogeneous  spheroid  and  to 
have  possessed  a  certain  small  viscosity,82  and  he  calculates 
the  internal  tidal  friction  in  such  a  mass  exposed  to  the  at- 
traction of  moon  and  sun,  and  the  consequences  which  these 
bodily  tides  have  produced.  He  finds  that  the  length  of 
our  day  and  month  have  greatly  increased,  that  the  moon's 
distance  has  likewise  augmented,  that  the  obliquity  of  the 
ecliptic  has  diminished,  that  a  large  amount  of  hypogene 
heat  has  been  generated  by  the  internal  tidal  friction,  and 
that  these  changes  may  all  have  transpired  within  compara- 
tively so  short  a  period  (57,000,000  years)  as  to  place  them 
quite  probably  within  the  limits  of  ordinary  geological  his- 
tory. According  to  his  estimate,  46,300,000  years  ago  the 
length  of  the  sidereal  day  was  fifteen  and  a  half  hours,  the 
moon's  distance  in  mean  radii  of  the  earth  was  46-8  as  com- 
pared with  60-4  at  the  present  time.  But  56,810,000  years 
back,  the  length  of  a  day  was  only  6|  hours,  or  less  than  a 
quarter  of  its  present  value,  the  moon's  distance  was  only 
nine  earth's  radii,  while  the  lunar  month  lasted  not  more 
than  about  a  day  and  a  half  (1-58),  or  tV  of  its  present  dura- 
tion. He  arrives  at  the  deduction  that  the  energy  lost  by 
internal  tidal  friction  in  the  earth's  mass  is  converted  into 
heat  at  such  a  rate  that  the  amount  lost  during  57,000,000 
years,  if  it  were  all  applied  at  once,  and  if  the  earth  had  the 
specific  heat  of  iron,  would  raise  the  temperature  of  the 
whole  planet's  mass  1,760°  Fahrenheit,  but  that  the  distribu- 
tion of  this  heat-generation  has  been  such  as  not  to  interfere 


83  The  degree  of  viscosity  assumed  is  such  that  "thirteen  and  a  half  tons  to 
the  square  inch  acting  for  twenty-four  hours  on  a  slab  an  inch  thick  displaces 
the  upper  surface  relatively  to  the  lower  through  one-tenth  of  an  inch.  It  is 
obvious,"  says  M'r.  Darwin,  "that  such  a  substance  as  this  would  be  called 
a  solid  in  ordinary  parlance,  and  in  the  tidal  problem  this  must  be  regarded  as 
a  very  small  viscosity."  Op.  cit.  p.  631. 


48  TEXT-BOOK    OF   GEOLOGY 

with  the  normal  augmentation  of  temperature  downward 
due  to  secular  cooling,  and  the  conclusion  drawn  therefrom 
by  Sir  William  Thomson.  Mr.  Darwin  further  concludes 
from  his  hypothesis  that  the  ellipticity  of  the  earth's  figure 
having  been  continually  diminishing,  "the  polar  regions 
must  have  been  ever  rising  and  the  equatorial  ones  falling, 
though  as  the  ocean  followed  these  changes,  they  might 
quite  well  have  left  no  geological  traces.  The  tides  must 
have  been  very  much  more  frequent  and  larger,  and  accord- 
ingly the  rate  of  oceanic  denudation  much  accelerated.  The 
more  rapid  alternation  of  day  and  night*8  would  probably 
lead  to  more  sudden  and  violent  storms,  and  the  increased 
rotation  of  -the  earth  would  augment  the  violence  of  the 
trade-winds,  which  in  their  turn  would  affect  oceanic  cur- 
rents." 3*  As  above  stated,  no  facts  yet  revealed  by  the 
geological  record  compel  the  admission  of  more  violent 
superficial  action  in  former  times  than  now.  But  though 
the  facts  do  not  of  themselves  lead  to  such  an  admission, 
it  is  proper  to  inquire  whether  any  of  them  are  hostile  to  it. 
It  will  be  shown  in  Book  VI.  that  even  as  far  back  as  early 
Palaeozoic  times,  that  is,  as  far  into  the  past  as  the  history 
of  organized  life  can  be  traced,  sedimentation  took  place 
very  much  as  it  does  now.  Sheets  of  fine  mud  and  silt  were 
pitted  with  rain  drops,  ribbed  with  ripple-marks,  and  fur- 
rowed by  crawling  worms,  exactly  as  they  now  are  on  the 
shores  of  any  modern  estuary.  These  surfaces  were  quietly 
buried  under  succeeding  sediment  of  a  similar  kind,  and  this 
for  hundreds  and  thousands  of  feet.  Nothing  indicates  vio- 
lence; all  the  evidence  favors  tranquil  deposit."  If,  there- 


83  According  to  his  calculation,  the  year  57,000,000  of  years  ago  contained 
1300  days  instead  of  365.  M  Op.  cit.  p.  532. 

35  Sir  R.  Ball  (Nature,  xxv.  1881,  pp.  79,  J03),  starting  from  Prof.  Darwin's 


COSMIC AL  ASPECTS  OF  GEOLOGY 

fore,  Mr.  Darwin's  hypothesis  be  accepted,  we  must  con- 
clude either  that  it  does  not  necessarily  involve  such  violent 
superficial  operations  as  he  supposes,  or  that  even  the  oldest 
sedimentary  formations  do  not  date  back  to  a  time  when  the 
influence  of  increased  rotation  could  make  itself  evident  in 
sedimentation,  that  is  to  say,  on  Mr.  Darwin's  hypothesis, 
the  most  ancient  fossiliferous  rocks  cannot  be  as  much  as 
57,000,000  years  old. 

§  8.  Climate  in  its  Geological  Relations. — In  subsequent 
parts  of  this  work  data  will  be  given  from  which  we  learn 
that  the  climates  of  the  earth  have  formerly  been  consider- 
ably different  from  those  which  at  present  prevail.  A  con- 
sideration of  the  history  of  the  solar  system  would  of  itself 
suggest  the  inference  that,  on  the  whole,  the  climates  of 
early  geological  periods  must  have  been  warmer.  The  sun's 
heat  was  greater,  probably  the  amount  of  it  received  by  the 
earth  was  likewise  greater,  while  there  would  be  for  some 
time  a  sensible  influence  of  the  planet's  own  internal  heat 
upon  the  general  temperature  of  the  whole  globe."  Al- 
though arguments  based  upon  the  probable  climatal  neces- 
sities of  extinct  species  and  genera  of  plants  and  animals 


data,  pushed  his  conclusions  to  such  an  extreme  as  to  call  in  the  agency  of  tides 
more  than  600  feet  high  in  early  geological  times.  In  repudiating  this  applica- 
tion of  his  results,  Mr.  Darwin  (Nature,  xxv.  p.  213)  employs  the  argument 
I  have  here  used  from  the  absence  of  any  evidence  of  such  tidal  action  in  the 
geological  formations,  and  from  the  indication,  on  the  contrary,  of  tranquil 
deposit. 

*  Lord  Kelvin  (Sir  William  Thomson)  believes  that  the  hypothesis  that  ter- 
restrial temperature  was  formerly  higher  by  reason  of  a  hotter  sun  "is  rendered 
almost  infinitely  probable  by  independent  physical  evidence  and  mathematical 
calculation."  (Trans.  Geol.  Soc.  Glasgow,  v.  p.  238.)  Prof.  Tail,  however,  has 
suggested,  that  the  former  greater  heat  of  the  sun  may  have  raised  such  vast 
clouds  of  absorbing  vapor  round  that  luminary  as  to  prevent  the  effective  amount 
of  radiation  of  heat  to  the  earth's  surface  from  being  greater  than  at  present; 
while  on  the  other  hand,  a  similar  supposition  may  be  made  with  reference  to 
the  greater  amount  of  vapor  which  increased  solar  radiation  would  raise  to  be 
condensed  in  the  earth's  atmosphere.  "Recent  Advances  in  Physical  Science," 
1876,  p.  174. 

GEOLOGY — Yol.  XXIX — 3 


50  TEXT-BOOK   OF   GEOLOGY 

must  be  used  with  extreme  caution,  it  may  be  asserted  with 
some  confidence  that  from  the  vast  areas  over  which  Palgeo- 
zoic  mollusks  have  been  traced,  alike  in  the  eastern  and  the 
western  hemispheres,  the  climates  of  the  globe  in  Palaeozoic 
time  were  probably  more  uniform  than  they  now  are.  There 
appears  to  have  been  a  gradual  lowering  of  the  general  tem- 
perature during  past  geological  time,  accompanied  by  a  ten- 
dency toward  greater  extremes  of  climate.  But  there  are 
propfs  also  that  at  longer  or  shorter  intervals  cold  cycles 
have  intervened.  The  Glacial  Period,  for  example,  pre- 
ceded our  own  time,  and  in  successive  geological  formations 
indications,  of  more  or  less  value,  have  been  found  that 
suggest  if  they  do  not  prove  a  former  prevalence  of  ice  in 
what  are  now  temperate  regions." 

Various  theories  have  been  proposed  in  explanation  of 
such  alternations  of  climate.  Some  of  these  have  appealed 
to  a  change  in  the  position  of  the  earth's  axis  relatively  to 
the  mass  of  the  planet  (ante,  §  5).  Others  have  been  based 
on  the  notion  that  the  earth  may  have  passed  through  hot 
and  cold  regions  of  space.  Others,  again,  have  called  in  the 
effects  of  terrestrial  changes,  such  as  the  distribution  of  land 
and  sea,  on  the  assumption  that  elevation  of  land  about  the 
poles  must  cool  the  temperature  of  the  globe,  while  eleva- 
tion round  the  equator  would  raise  it.88  But  the  changes  of 
temperature  appear  to  have  affected  the  whole  of  the  earth's 
surface,  while  there  is  not  only  no  proof  of  any  such  enor- 
mous vicissitudes  in  physical  geography  as  would  be  re- 
quired, but  good  grounds  for  believing  that  the  present  ter- 


31  Consult  a  suggestive  paper  by  the  late  Dr.  M.  Neumayr,  Nature,  xlii.  (1890) 
p.  148. 

38  In  Lyell's  "Principles  of  Geology,"  this  doctrine  of  the  influence  of  geo- 
graphical changes  is  maintained. 


COSMICAL  ASPECTS  OF  GEOLOGY        51 

restrial  and  oceanic  areas  have  remained,  on  the  whole,  on 
the  same  sites  from  very  early  geological  time.  Moreover, 
as  evidence  has  accumulated  in  favor  of  periodic  alterna- 
tions of  climate,  the  conviction  has  been  strengthened  that 
no  mere  local  changes  could  have  sufficed,  but  that  secular 
variations  in  climate  must  be  assigned  to  some  general  and 
probably  recurring  cause. 

By  degrees,  geologists  accustomed  themselves  to  the  be- 
lief that  the  cold  of  the  Glacial  Period  was  not  due  to  mere 
terrestrial  changes,  but  was  to  be  explained  somehow  as  the 
result  of  cosmical  causes.  Of  various  suggestions  as  to  the 
probable  nature  and  operation  of  these  causes,  one  deserves 
careful  consideration  —  change  in  the  eccentricity  of  the 
earth's  orbit.  Sir  John  HerscheP*  pointed  out  many  years 
ago  that  the  direct  effect  of  a  high  condition  of  eccentricity 
is  to  produce  an  unusually  cold  winter,  followed  by  a  corre- 
spondingly hot  summer,  in  the  hemisphere  whose  winter 
occurs  in  aphelion,  while  an  equable  condition  of  climate 
at  the  same  time  prevails  on  the  opposite  hemisphere.  But 
both  hemispheres  must  receive  precisely  the  same  amount 
of  solar  heat,  because  the  deficiency  of  heat,  resulting  from 
the  sun's  greater  distance  during  one  part  of  the  year,  is 
exactly  compensated  by  the  greater  length  of  that  season. 
Sir  John  Herschel  even  considered  that  the  direct  effects 
of  eccentricity  must  thus  be  nearly  neutralized.40  As 
a  like  verdict  was  afterward  given  by  Arago,  Humbolch, 
and  others,  geologists  were  satisfied  that  no  important 
change  of  climate  could  be  attributed  to  change  of  eccen- 
tricity. 

The  late  Dr.  James  Croll,  as  far  back  as  the  year  1864, 

39  Trans.  Geol.  Soc.  vol.  iii.  p.  293  (2d  series). 

40  "Cabinet  Cyclopaedia,"  sec.  315;  "Outlines  of  Astronomy,"  sec.  368. 


62  TEXT-BOOK   OF   GEOLOGY 

made  an  important  suggestion  in  this  matter,  and  subse- 
quently worked  out  an  elaborate  development  of  the  whole 
subject  of  the  physical  causes  on  which  climate  depends.41 
He  was  good  enough  to  draw  up  the  following  abstract  of 
them  for  former  editions  of  the  present  work. 

"Assuming  the  mean  distance  of  the  sun  to  be  92,400,000 
miles,  then  when  the  eccentricity  is  at  its  superior  limit, 
•07775,  the  distance  of  the  sun  from  the  earth,  when  the 
latter  is  in  the  aphelion  of  its  orbit,  is  no  less  than  99,584,100 
miles,  and  when  in  the  perihelion  it  is  only  85,215, 900'miles. 
The  earth  is,  therefore,  14,368,200  miles  further  from  the 
sun  in  the  former  than  in  the  latter  position.  The  direct 


N.  Winter  Solstice  in  Aphelion  N.  Winter  Solstice  in  Perihelion 

Fig.  1.— Eccentricity  of  the  Earth's  Orbit  in  Relation  to  Climate 

heat  of  the  sun  being  inversely  as  the  square  of  the  distance, 
it  follows  that  the  amount  of  heat  received  by  the  earth  in 
these  two  positions  will  be  as  19  to  26.  The  present  eccen- 
tricity being  -0168,  the  earth's  distance  during  our  northern 
winter  is  90,847,680  miles.  Suppose  now  that,  from  the 
precession  of  the  equinoxes,  winter  in  our  northern  hemi- 
sphere should  happen  when  the  earth  is  in  the  aphelion  of 
its  orbit,  at  the  time  that  the  orbit  is  at  its  greatest  eccen- 

41  Phil.  Mag.  xxviii.  (1864),  p.  121.  His  researches  will  be  found  in  detail 
in  his  volume  "Climate  and  Time,"  1875,  and  his  later  work  "Discussions  on 
Climate  and  Cosmology." 


COSMIC AL   ASPECTS    OF   GEOLOGY  53 

tricity;  the  earth  would  then  be  8,736,420  miles  further  from 
the  sun  in  winter  than  it  is  at  present.  The  direct  heat  of 
the  sun  would  therefore,  during  winter,  be  one-fifth  less  and 
during  summer  one-fifth  greater  than  now.  This  enormous 
difference  would  necessarily  affect  the  climate  to  a  very 
great  extent.  Were  the  winters  under  these  circumstances 
to  occur  when  the  earth  was  in  the  perihelion  of  its  orbit, 
the  earth  would  then  be  14,368,200  miles  nearer  the  sun  in 
winter  than  in  summer.  In  this  case  the  difference  between 
winter  and  summer  in  our  latitudes  would  be  almost  annihi- 
lated. But  as  the  winters  in  the  one  hemisphere  correspond 
with  the  summers  in  the  other,  it  follows  that  while  the  one 
hemisphere  would  be  enduring  the  greatest  extremes  of  sum- 
mer heat  and  winter  cold,  the  other  would  be  enjoying  per- 
petual summer. 

"It  is  quite  true  that,  whatever  may  be  the  eccentricity 
of  the  earth's  orbit,  the  two  hemispheres  must  receive  equal 
quantities  of  heat  per  annum;  for  proximity  to  the  sun  is 
exactly  compensated  by  the  effect  of  swifter  motion.  The 
total  amount  of  heat  received  from  the  sun  between  the  two 
equinoxes  is,  therefore,  the  same  in  both  halves  of  the  year, 
whatever  the  eccentricity  of  the  earth's  orbit  may  be.  For 
example,  whatever  extra  heat  the  southern  hemisphere  may 
at  present  receive  per  day  from  the  sun  during  its  summer 
months,  owing  to  greater  proximity  to  the  sun,  is  exactly 
compensated  by  a  corresponding  loss  arising  from  the  short- 
ness of  the  season;  and,  on  the  other  hand,  whatever  defi- 
ciency of  heat  we  in  the  northern  hemisphere  may  at  present 
have  per  day  during  our  summer  half-year,  in  consequence 
of  the  earth *s  distance  from  the  sun,  is  also  exactly  compen- 
sated by  a  corresponding  length  of  season. 

"It  is  well  known,  however,  that  those  simple  changes 
in  the  summer  and  winter  distances  would  not  alone  produce 
a  glacial  epoch,  and  that  physicists,  confining  their  attention 
to  the  purely  astronomical  effects,  were  perfectly  correct  in 
affirming  that  no  increase  of  eccentricity  of  the  earth's  orbit 
could  account  for  that  epoch.  But  the  important  fact  was 
overlooked  that,  although  the  glacial  epoch  could  not  result 
directly  from  an  increase  of  eccentricity,  it  might  neverthe- 
less do  so  indirectly  from  physical  agents  that  were  brought 
into  operation  as  a  result  of  an  increase  of  eccentricity. 
The  following  is  an  outline  of  what  these  physical  agents 
were,  how  they  were  brought  into  operation,  'and  the  way 
in  which  they  may  have  led  to  the  glacial  epoch. 


54  TEXT-BOOK   OF   GEOLOGY 

"With  the  eccentricity  at  its  superior  limit  and  the  win- 
ter occurring  in  the  aphelion,  the  earth  would,  as  we  have 
seen,  be  8,736,420  miles  further  from  the  sun  during  that 
season  than  at  present.  The  reduction  in  the  amount  of 
heat  received  from  the  sun,  owing  to  his  increased  distance, 
would  lower  the  midwinter  temperature  to  an  enormous  ex- 
tent. In  temperate  regions  the  greater  portion  of  the  moist- 
ure of  the  air  is  at  present  precipitated  in  the  form  of  rain, 
and  the  very  small  portion  which  falls  as  snow  disappears 
in  the  course  of  a  few  weeks  at  most.  But  in  the  circum- 
stances under  consideration,  the  mean  winter-temperature 
would  be  lowered  so  much  below  the  freezing-point  that 
what  now  falls  as  rain  during  that  season,  would  then  fall 
as  snow.  This  is  not  all;  the  winters  would  then  not  only 
be  cooler  than  now,  but  they  would  also  be  much  longer. 
At  present  the  winters  are  nearly  eight  days  shorter  than  the 
summers;  but  with  the  eccentricity  at  its  superior  limit  and 
the  winter  solstice  in  aphelion,  the  length  of  the  winters 
would  exceed  that  of  the  summers  by  no  fewer  than  thirty- 
six  days.  The  lowering  of  the  temperature  and  the  length- 
ening of  the  winter  would  both  tend  to  the  same  effect,  viz. 
to  increase  the  amount  of  snow  accumulated  during  the  win- 
ter; for,  other  things  being  equal,  the  longer  the  snow-ac- 
cumulating period,  the  greater  the  accumulation.  It  may 
be  remarked,  however,  that  the  absolute  quantity  of  heat 
received  during  winter  is  not  affected  by  the  decrease  in  the 
sun's  heat,  for  the  additional  length  of  the  season  compen- 
sated for  this  decrease."  As  regards  the  absolute  amount 
of  heat  received,  increase  of  the  sun's  distance  and  length- 
ening of  the  winter  are  compensatory,  but  not  so  in  regard 
to  the  amount  of  snow  accumulated.  The  consequence  of 
this  state  of  things  would  be  that,  at  the  commencement 
of  the  short  summer,  the  ground  would  be  covered  with  the 
winter's  accumulation  of  snow.  Again,  the  presence  of  so 
much  snow  would  lower  the  summer  temperature,  and  pre- 
vent to  a  great  extent  the  melting  of  the  snow. 

"There  are  three  separate  ways  whereby  accumulated 
masses  of  snow  and  ice  tend  to  lower  the  summer  tempera- 
ture, viz. : 

"First,  By  means  of  direct  radiation.  No  matter  what 
the  intensity  of  the  sun's  rays  may  be,  the  temperature  of 

42  When  the  eccentricity  is  at  its  superior  limit,  the  absolute  quantity  of  heat 
received  by  the  earth  during  the  year  is,  however,  about  one  three-hundredth 
part  greater  than  at  present.  But  this  does  not  affect  the  question  at  issue. 


COSMICAL  ASPECTS  OF  GEOLOGY         55 

enow  and  ice  can  never  rise  above  32°.  Hence,  the  pres- 
ence of  snow  and  ice  tends  by  direct  radiation  to  lower  the 
temperature  of  all  surrounding  bodies  to  32°.  In  Green- 
land, a  country  covered  with  snow  and  ice,  the  pitch  has 
been  seen  to  melt  on  the  side  of  a  ship  exposed  to  the  direct 
rays  of  the  sun,  while  at  the  same  time  the  surrounding  air 
was  far  below  the  freezing-point;  a  thermometer  exposed  to 
the  direct  radiation  of  the  sun  has  been  observed  to  stand 
above  100°,  while  the  air  surrounding  the  instrument  was 
actually  12°  below  the  freezing-point.  A  similar  experience 
has  been  recorded  by  travellers  on  the  snow-fields  of  the 
Alps.  These  results,  surprising  as  they  no  doubt  appear, 
are  what  we  ought  to  expect  under  the  circumstances.  Per- 
fectly dry  air  seems  to  be  nearly  incapable  of  absorbing 
radiant  heat.  The  entire  radiation  passes  through  it  almost 
without  any  sensible  absorption.  Consequently  the  pitch 
on  the  side  of  the  ship  may  be  melted,  or  the  bulb  of  the 
thermometer  raised  to  a  high  temperature  by  the  direct  rays 
of  the  sun,  while  the  surrounding  air  remains  intensely  cold. 
The  air  is  cooled  by  contact  with  the  snow-covered  ground, 
but  is  not  heated  by  the  radiation  from  the  sun. 

"When  the  air  is  charged  with  aqueous  vapor,  a  similar 
cooling  effect  also  takes  place,  but  in  a  slightly  different 
way.  Air  charged  with  aqueous  vapor  is  a  good  absorber 
of  radiant  heat,  but  it  can  only  absorb  those  rays  which 
agree  with  it  in  period.  It  so  happens  that  rays  from  snow 
and  ice  are,  of  all  others,  those  which  it  absorbs  best.  The 
humid  air  will  absorb  the  total  radiation  from  the  snow 
and  ice,  but  it  will  allow  the  greater  part  of,  if  not  nearly 
all,  the  sun's  rays  to  pass  unabsorbed.  But  during  the 
day,  when  the  sun  is  shining,  the  radiation  from  the  snow 
and  ice  to  the  air  is  negative;  that  is,  the  snow  and  ice 
cool  the  air  by  radiation.  The  result  is,  the  air  is  cooled 
by  radiation  from  the  snow  and  ice  (or  rather,  we  should 
say,  to  the  snow  and  ice)  more  rapidly  than  it  is  heated  by 
the  sun;  and  as  a  consequence,  in  a  country  like  Greenland, 
covered  with  an  icy  mantle,  the  temperature  of  the  air,  even 
during  summer,  seldom  rises  above  the  freezing-point. 
Snow  is  a  good  reflector,  but  as  simple  reflection  does  not 
change  the  character  of  the  rays,  they  would  not  be  ab- 
sorbed by  the  air,  but  would  pass  into  stellar  space.  Were 
it  not  for  the  ice,  the  summers  of  North  Greenland,  owing 
to  the  continuance  of  the  sun  above  the  horizon,  would  be 
as  warm  as  those  of  England ;  but  instead  of  this,  the  Green- 


56  TEXT-BOOK   OF   GEOLOGY 

land  summers  are  colder  than  our  winters.  Cover  India 
with  an  ice  sheet,  and  its  summers  would  be  colder  than 
those  of  England. 

"  Second,  Another  cause  of  the  cooling  effect  is  that  the 
rays  which  fall  on  snow  and  ice  are  to  a  great  extent  re- 
flected back  into  space.  But  those  that  are  not  reflected, 
but  absorbed,  do  not  raise  the  temperature,  for  they  disap- 
pear in  the  mechanical  work  of  melting  the  ice.  For  what- 
soever may  be  the  intensity  of  the  sun's  heat,  the  surface 
of  the  ground  will  be  kept  at  82°  so  long  as  the  snow  and 
ice  remain  unmelted. 

"Third,  Snow  and  ice  lower  the  temperature  by  chilling 
the  air  and  condensing  the  vapor  into  thick  fogs.  The  great 
strength  of  the  sun's  rays  during  summer,  due  to  his  near- 
ness at  that  season,  would,  in  the  first  place,  tend  to  produce 
an  increased  amount  of  evaporation.  But  the  presence  of 
snow-clad  mountains  and  an  icy  sea  would  chill  the  atmos- 

Ehere  and  condense  the  vapor'  into  thick  fogs.  The  thick 
)gs  and  cloudy  sky  would  effectually  prevent  the  sun's 
rays  from  reaching  the  earth,  and  the  snow,  in  consequence, 
would  remain  unmelted  during  the  entire  summer.  In  fact, 
we  have  this  very  condition  of  things  exemplified  in  some 
of  the  islands  01  the  Southern  Ocean  at  the  present  day. 
Sandwich  Land,  which  is  in  the  same  parallel  of  latitude 
as  the  north  of  Scotland,  is  covered  with  ice  and  snow  the 
entire  summer;  and  in  the  island  of  South  Georgia,  which 
is  in  the  same  parallel  as  the  centre  of  England,  the  per- 
petual snow  descends  to  the  very  sea-beach.  Captain  Sir 
James  Eoss  found  the  perpetual  snow  at  the  sea-level  at 
Admiralty  Inlet,  South  Shetland,  in  lat.  64°;  and  while 
near  this  place  the  thermometer  in  the  very  middle  of 
summer  fell  at  night  to  23°  F.  The  reduction  of  the  sun's 
heat  and  lengthening  of  the  winter,  which  would  take  place 
when  the  eccentricity  is  near  to  its  superior  limit  and  the 
winter  in  aphelion,  would  in  this  country  produce  a  state 
of  things  perhaps  as  bad  as,  if  not  worse  than,  that  which 
at  present  exists  in  South  Georgia  and  South  Shetland. 

"The  cause  which  above  all  others  must  tend  to  produce 
great  changes  of  climate,  is  the  deflection  of  great  ocean  cur- 
rents. A  high  condition  of  eccentricity  tends,  we  have  seen, 
to  produce  an  accumulation  of  snow  and  ice  on  the  hemi- 
sphere whose  winters  occur  in  aphelion.  The  accumulation 
of  snow,  in  turn,  tends  to  lower  the  summer  temperature, 
cut  off  the  sun's  rays,  and  retard  the  melting  of  the  snow. 


COSMICAL  ASPECTS  OF  GEOLOGY         57 

In  short,  it  tends  to  produce,  on  that  hemisphere,  a  state 
of  glaciation.  Exactly  opposite  effects  take  place  on  the 
other  hemisphere,  which  has  its  winter  in  perihelion. 
There  the  shortness  of  the  winters,  combined  with  the 
high  temperature  arising  from  the  nearness  of  the  sun, 
tends  to  prevent  the  accumulation  of  snow.  The  general 
result  is  that  the  one  hemisphere  is  cooled  and  the  other 
heated.  This  state  of  things  now  brings  into  play  the 
agencies  which  lead  to  the  deflection  of  the  Q-uli  Stream 
and  other  great  ocean  currents. 

"Owing  to  the  great  difference  between  the  temperature 
of  the  equator  and  the  poles,  there  is  a  constant  flow  of  air 
from  the  poles  to  the  equator.  It  is  to  this  that  the  trade- 
winds  owe  their  existence.  Now,  as  the  strength  of  these 
winds  will,  as  a  general  rule,  depend  upon  the  difference 
of  temperature  that  may  exist  between  the  equator  and 
higher  latitudes,  it  follows  that  the  trades  on  the  cold 
hemisphere  will  be  stronger  than  those  on  the  warm.  When 
the  polar  and  temperate  regions  of  the  one  hemisphere  are 
covered  to  a  large  extent  with  snow  and  ice,  the  air,  as  we 
have  just  seen,  is  kept  almost  at  the  freezing-point  during 
both  summer  and  winter.  The  trades  on  that  hemisphere 
will,  of  necessity,  be  exceedingly  powerful;  while  on  the 
other  hemisphere,  where  there  is  comparatively  little  snow 
or  ice,  and  the  air  is  warm,  the  trades  will  consequently  be 
weak.  Suppose  now  the  northern  hemisphere  to  be  the  cold 
one.  The  northeast  trade-winds  of  this  hemisphere  will  far 
exceed  in  strength  the  southeast  trade-winds  of  the  southern 
hemisphere.  The  median  line  between  the  trades  will  con- 
sequently lie  to  a  very  considerable  distance  to  the  south 
of  the  equator.  We  have  a  good  example  of  this  at  the 
present  day.  The  difference  of  temperature  between  the  two 
hemispheres  at  present  is  but  trifling  to  what  it  would  be  in 
the  case  under  consideration;  yet  we  find  that  the  southeast 
trades  of  the  Atlantic  blow  with  greater  force  than  the 
northeast  trades,  sometimes  extending  to  10°  or  15°  N.  lat., 
whereas  the  northeast  trades  seldom  blow  south  of  the 
equator.  The  effect  of  the  northern  trades  blowing  across 
the  equator  to  a  great  distance  will  be  to  impel  the  warm 
water  of  the  tropics  over  into  the  Southern  Ocean.  But  this 
is  not  all;  not  only  would  the  median  line  of  the  trades  be 
shifted  southward,  but  the  great  equatorial  currents  of  the 
globe  would  also  be  shifted  southward. 

"Let  us  now  consider  how  this  would  affect  the  Gulf 


58  TEXT-BOOK    OF   GEOLOGY 

Stream.  The  South  American  continent  is  shaped  some- 
what in  the  form  of  a  triangle,  with  one  of  its  angular 
corners,  called  Cape  St.  Roque,  pointing  eastward.  The 
equatorial  current  of  the  Atlantic  impinges  against  this 
corner;  but  as  the  greater  portion  of  the  current  lies  a  little 
to  the  north  of  the  corner,  it  flows  westward  into  the  Gulf 
of  Mexico  and  forms  the  Gulf  Stream.  A  considerable 
portion  of  the  water,  however,  strikes  the  land  to  the  south 
of  the  cape,  and  is  deflected  along  the  shore  of  Brazil  into 
the  Southern  Ocean,  forming  what  is  known  as  the  Brazilian 
current.  Now,  it  is  obvious  that  the  shifting  of  the  equa- 
torial current  of  the  Atlantic  only  a  few  degrees  to  the  south 
of  its  present  position — a  thing  which  would  certainly  take 
place  under  the  conditions  which  we  have  been  detailing — 
would  turn  the  entire  current  into  the  Brazilian  branch,  and 
instead  of  flowing  chiefly  into  the  Gulf  of  Mexico,  as  at 
present,  it  would  all  flow  into  the  Southern  Ocean,  and  the 
Gulf  Stream  would  consequently  be  stopped.  The  stoppage 
of  the  Gulf  Stream,  combined  with  all  those  causes  which 
we  have  just  been  considering,  would  place  Europe  under 
a  glacial  condition,  while  at  the  same  time  the  temperature 
of  the  Southern  Ocean  would,  in  consequence  of  the  enor- 
mous quantity  of  warm  water  received,  have  its  temperature 
(already  high  from  other  causes)  raised  enormously.  And 
what  holds  true  in  regard  to  the  currents  of  the  Atlantic 
holds  also  true,  though  perhaps  not  to  the  same  extent,  of 
the  currents  of  the  Pacific. 

"If  the  breadth  of  the  Gulf  Stream  be  taken  at  50  miles, 
its  depth  at  1000  feet,  its  mean  velocity  at  2  statute  miles 
an  hour,  the  temperature  of  the  water  when  it  leaves  the 
gulf  at  65°,  and  the  return  current  at  40°  F.,48  then,  the 
(quantity  of  heat  conveyed  into  the  Atlantic  by  this  stream 
is  equal  to  one-fourth  of  all  the  heat  received  from  the  sun 
by  that  ocean  from  the  Tropic  of  Cancer  to  the  Arctic 
Circle."  From  principles  discussed  at  considerable  length 


48  Sir  Wyville  Thomson  states  that  in  May,  1873,  the  "Challenger"  expedi- 
tion found  the  Gulf  Stream,  at  the  point  where  it  was  crossed,  to  be  about  sixty 
miles  in  width,  100  fathoms  deep,  and  flowing  at  the  rate  of  three  knots  per 
hour.  This  makes  the  volume  of  the  stream  one-fifth  greater  than  the  above 
estimate. 

44  The  quantity  of  heat  conveyed  by  the  Gulf  Stream  for  distribution  is  equal 
to  77,479,650,000,000,000,000  foot-pounds  per  day.  The  quantity  received 
from  the  sun  by  the  North  Atlantic  is  310,923,000,000,000,000,000  foot-pounds. 
"Climate  and  Time,"  chap.  ii. 


COSMIC 'AL    ASPECTS    OF   GEOLOGY  59 

in  'Climate  and  Time'  it  is  shown  that,  but  for  the  Gulf 
Stream  and  other  currents,  London  would  have  a  mean 
annual  temperature  40°  lower  than  at  present. 

"But  there  is  still  another  cause  which  must  be  noticed 
— a  strong  undercurrent  of  air  from  the  north  implies  an 
equally  strong  upper  current  to  the  north.  Now  if  the 
effect  of  the  undercurrent  would  be  to  impel  the  warm 
water  at  the  equator  to  the  south,  the  effect  of  the  upper 
current  would  be  to  carry  the  aqueous  vapor  formed  at  the 
equator  to  the  north ;  the  upper  current,  on  reaching  the 
snow  and  ice  of  temperate  regions,  would  deposit  its  moist- 
ure in  the  form  of  snow;  so  that  it  is  probable  that,  not- 
withstanding the  great^  cold  of  the  glacial  epoch,  the  quan- 
tity of  snow  falling  m  the  northern  regions  would  be 
enormous.  This  would  be  particularly  the  case  during 
summer,  when  the  earth  would  be  in  the  perihelion  and 
the  heat  at  the  equator  great.  The  equator  would  be  the 
furnace  where  evaporation  would  take  place,  and  the  snow 
and  ice  of  temperate  regions  would  act  as  a  condenser. 

"The  foregoing  considerations,  as  well  as  many  others 
which  might  be  stated,  lead  to  the  conclusion  that,  in  order 
to  raise  the  mean  temperature  of  the  globe,  water  should  be 
placed  along  the  equator,  and  not  land,  as  was  contended 
by  Sir  Charles  Lyell  and  others.  For  if  land  be  placed  at 
the  equator,  the  possibility  of  conveying  the  sun's  neat  from 
the  equatorial  regions  by  means  of  ocean  currents  is  pre- 
vented. ' '  *• 

The  astronomical  theory  in  explanation  of  former  great 
differences  of  terrestrial  climate  has  recently  been  illustrated 
and  enforced  by  Sir  Robert  Ball,  who,  while  strengthening 
the  general  arguments  in  its  favor,  especially  insists  upon 
the  existence  of  an  important  law  in  the  distribution  of 
solar  heat  on  the  earth's  surface,  which  he  thinks  has  been 
hitherto  overlooked.  He  remarks  that  the  original  state- 

45  That  climate,  however,  may  be  considerably  affected  by  changes,  such  as 
are  known  to  have  taken  place  in  the  distribution  of  land  and  sea,  must  be 
frankly  conceded.  This  has  been  recently  cogently  argued  by  Mr.  Wallace 
in  his  "Island  Life,"  1880.  Mr.  Croll's  views,  summarized  above,  have  been 
adversely  criticised  by  Prof.  Newcombe,  for  whose  papers  and  Dr.  Croll's  re- 
plies see  Amer.  Journ.  Science,  1876,  1883,  1884,  and  the  work  by  the  latter 
writer,  "Discussions  on  Climate  and  Cosmology,"  already  referred  to. 


60  TEXT-BOOK    OF   GEOLOGY 

ment  of  Sir  John  Herschel  that  the  heat  received  by  the 
earth  from  the  sun  is  equally  divided  between  the  winter 
and  summer  seasons  has  given  rise  to  an  entirely  erroneous 
impression.  Although  "it  is  certainly  true  that  during  the 
summer  in  one  hemisphere  the  heat  received  on  the  whole 
earth  is  equal  to  the  heat  received  on  the  whole  earth  dur- 
ing the  ensuing  winter  on  the  same  hemisphere,"  yet  on 
any  given  hemisphere  almost  twice  as  much  heat  can  be 
demonstrated  to  be  received  during  summer  as  during 
winter.4'  The  law  is  thus  stated:  "Of  the  total  amount 
of  heat  received  from  the  sun  on  a  hemisphere  of  the 
earth'  in  the  course  of  a  year,  63  per  cent  is  received 
during  the  summer,  and  37  per  cent  is  received  during 
the  winter."47  It  is  obvious  that  while,  under  the  opera- 
tion of  this  law,  the  total  amount  of  heat  received  and 
the  ratio  of  its  distribution  between  summer  and  winter 
would  remain  unchanged,  enormous  differences  in  terres- 
trial climate  might  result  according  as  the  seasons  varied 
in  length  with  changes  in  the  eccentricity  of  the  orbit. 

Inter-Glacial  Periods.  —  Allusion  has  already 
been  made  to  the  accumulating  evidence  that  changes  of 
climate  have  been  recurrent,  and  to  the  deduction  from 
this  alternation  or  periodicity  that  they  have  probably 
been  due  to  some  general  or  cosmical  cause.  Dr.  Croll 
ingeniously  showed  that  every  long  cold  period  arising 
in  each  hemisphere  from  the  circumstances  sketched  in 
the  preceding  pages,  must  have  been  interrupted  by  sev- 
eral shorter  warm  periods. 

"When  the  one  hemisphere,"  he  says,  "is  under  glacia- 
tion,  the  other  is  enjoying  a  warm  and  equable  climate. 

46  "The  Cause  of  an  Ice  Age,"  London,  1891,  p.  120.         41  Ibid.  p.  90. 


COSMIC AL   ASPECTS    OF   GEOLOGY  61 

But,  owing  to  the  precession  of  the  equinoxes,  the  condi- 
tion of  things  on  the  two  hemispheres  must  be  reversed 
every  10,000  years  or  so.  When  the  solstice  passes  the 
aphelion,  a  contrary  process  commences;  the  snow  and 
ice  gradually  begin  to  diminish  on  the  cold  hemisphere 
and  to  make  their  appearance  on  the  other  hemisphere. 
The  glaciated  hemisphere  turns  by  degrees  warmer,  and 
the  warm  hemisphere  colder,  and  this  continues  to  go  on 
for  a  period  of  ten  or  twelve  thousand  years,  until  the 
winter  solstice  reaches  the  perihelion.  By  this  time  the 
conditions  of  the  two  hemispheres  have  been  reversed; 
the  formerly  glaciated  hemisphere  has  now  become  the 
warm  one,  and  the  warm  hemisphere  the  glaciated.  The 
transference  of  the  ice  from  the  one  hemisphere  to  the  other 
continues  as  long  as  the  eccentricity  remains  at  a  high 
value.  It  is  probable  that,  during  the  warm  inter-glacial 
periods,  Greenland  and  the  Arctic  regions  would  be  com- 
paratively free  from  snow  and  ice,  and  enjoying  a  temperate 
and  equable  climate. ' ' 


62  TEXT-BOOK    OF   GEOLOGY 


BOOK   II 

GEOGNOSY 

AN  INVESTIGATION  OF  THE  MATERIALS  OF  THE  EARTH'S  SUBSTANCE 

PART  I. — A  GENERAL  DESCRIPTION  OF  THE  PARTS 
OF  THE  EARTH 

A  DISCUSSION  of  the  geological  changes  which  our 
planet  has   undergone  ought   to   be   preceded   by   a 
study  of  the  materials  of  which  the  planet  consists. 
This  latter  branch  of  inquiry  is  termed  Geognosy. 

"Viewed  in  a  broad  way,  the  earth  may  be  considered  as 
consisting  of  (1)  two  envelopes — an  outer  one  of  gas  (atmos- 
phere), completely  surrounding  the  planet,  and  an  inner  one 
of  water  (hydrosphere),  covering  about  three-fourths  of  the 
globe;  and  (2)  a  globe  (lithosphere),  cool  and  solid  on  its 
surface,  but  possessing  a  high  internal  temperature. 

I. — The  Envelopes — Atmosphere  and  Hydrosphere 

It  is  certain  that  the  present  gaseous  and  liquid  enve- 
lopes of  the  planet  form  only  a  portion  of  the  original  mass 
of  gas  and  water  with  which  the  globe  was  invested.  Fully 
a  half  of  the  outer  shell  or  crust  of  the  earth  consists  of  oxy- 
gen, which,  there  can  be  no  doubt,  once  existed  in  the  at- 
mosphere. The  extent,  likewise,  to  which  water  has  been 
abstracted  by  minerals  is  almost  incredible.  It  has  been 
estimated  that  already  one-third  of  the  whole  mass  of  the 
ocean  has  been  thus  absorbed.  Eventually  the  condition  of 
the  planet  will  probably  resemble  that  of  the  moon — a  globe 
without  air,  or  water,  or  life  of  any  kind. 


GEOGNOSY  68 

1.  The  Atmosphere.— The  gaseous  envelope  to  which 
the  name  of  atmosphere  is  given,  extends  to  a  distance 
of  perhaps  500  or  600  miles  from  the  earth's  surface,  pos- 
sibly in  a  state  of  extreme  tenuity  to  a  still  greater 
height.  But  its  thickness  must  necessarily  vary  with 
latitude  and  changes  in  atmospheric  pressure.  The  layer 
of  air  lying  over  the  poles  is  not  so  deep  as  that  which 
surrounds  the  equator. 

Many  speculations  have  been  made  regarding  the  chemi- 
cal composition  of  the  atmosphere  during  former  geological 
periods.  There  can  indeed  be  no  doubt  that  it  must  origi- 
nally ha.ve  differed  very  greatly  from  its  present  condition. 
Besides  the  abstraction  of  the  oxygen  which  now  forms  fully 
a  half  of  the  outer  crust  of  the  earth,  the  vast  beds  of  coal 
found  all  over  the  world,  in  geological  formations  of  many 
different  ages,  doubtless  represent  so  much  carbon -dioxide 
(carbonic  acid)  once  present  in  the  air.  According  to  Sterry 
Hunt,  the  amount  of  carbonic  acid  absorbed  in  the  process 
of  rock-decay,  and  now  represented  in  the  form  of  carbo- 
nates in  the  earth's  crust,  probably  equals  two  hundred 
times  the  present  volume  of  the  entire  atmosphere.1  The 
chlorides  in  the  sea,  likewise,  were  probably  carried  down 
out  of  the  atmosphere  in  the  primitive  condensation  of 
aqueous  vapor.  It  has  often  been  stated  that,  during  the 
Carboniferous  period,  the  atmosphere  must  have  been 
warmer  and  with  more  aqueous  vapor  and  carbon-diox- 
ide in  its  composition  than  at  the  present  day,  to  admit 
of  so  luxuriant  a  flora  as  that  from  which  the  coal-seams 
were  formed.  There  seems,  however,  to  be  at  present  no 
method  of  arriving  at  any  certainty  on  this  subject. 

1  Brit.  Aasoc.  Rep.  1878,  Sects,  p.  544. 


64  TEXT-BOOK   OF   GEOLOGY 

As  now  existing,  the  atmosphere  is  considered  to  be  nor- 
mally a  mechanical  mixture  of  nearly  4  volumes  of  nitrogen 
and  1  of  oxygen  (N79-4,  O20-6),  with  minute  proportions  of 
carbon-dioxide  and  water- vapor  and  still  smaller  quantities 
of  ammonia  and  the  powerful  oxidizing  agent,  ozone. 
These  quantities  are  liable  to  some  variation  according  to 
locality.  The  mean  proportion  of  carbon-dioxide  is  about 
3-5  parts  in  every  10,000  of  air.  In  the  air  of  streets  and 
houses  the  proportion  of  oxygen  diminishes,  while  that  of 
carbon- dioxide  increases.  According  to  the  researches  of 
Angus  Smith,  very  pure  air  should  contain  not  less  than 
20-99  per  cent  of  oxygen,  with  0-030  of  carbon-dioxide;  but 
he  found  impure  air  in  Manchester  to  have  only  20-21  of 
oxygen,  while  the  proportion  of  carbon-dioxide  in  that  city 
during  fog  was  ascertained  to  rise  sometimes  to  0-0679,  and 
in  the  pit  of  the  theatre  to  the  very  large  amount  of  0-2734. 
As  plants  absorb  carbon-dioxide  during  the  day  and  give  it 
off  at  night,  the  quantity  of  this  gas  in  the  atmosphere  oscil- 
lates between  a  maximum  at  night  and  a  minimum  during 
the  day.  During  the  part  of  the  year  when  vegetation  is 
active,  it  is  believed  that  there  is  at  least  10  per  cent  more 
carbonic  acid  in  the  air  of  the  open  country  at  night  than  in 
the  day.*  Small  as  the  normal  percentage  of  this  gas  in  the 
air  may  seem,  yet  the  total  amount  of  it  in  the  whole  atmos- 
phere probably  exceeds  what  would  be  disengaged  if  all  the 
vegetable  and  animal  matter  on  the  earth's  surface  were 
burned. 

The  other  substances  in  the  air  are  gases,  vapors, 
and  solid  particles.  Of  these  by  much  the  most  impor- 
tant is  the  vapor  of  water,  which  is  always  present,  but 


9  Prof.  G.  F.  Armstrong.     Proc.  Roy.  Soc.  xxx.  (1880),  p.  343. 


GEOGNOSY  65 

in  very  variable  amount  according  to  temperature.'  It 
is  this  vapor  which  chiefly  absorbs  radiant  heat.4  It  con- 
denses into  dew,  rain,  hail,  and  snow.  In  assuming  a 
visible  form,  and  descending  through  the  atmosphere,  it 
takes  up  a  minute  quantity  of  air,  and  of  the  different  sub- 
stances which  the  air  may  contain.  Being  caught  by  the 
rain,  and  held  in  solution  or  suspension,  these  substances 
can  be  best  examined  by  analyzing  rain-water.  In  this  way, 
the  atmospheric  gases,  ammonia,  nitric,  sulphurous,  and 
sulphuric  acids,  chlorides,  various  salts,  solid  carbon,  inor- 
ganic dust,  and  organic  matter  have  been  detected.  The 
fine  microscopic  dust  so  abundant  in  the  air  is  no  doubt  for 
the  most  part  due  to  the  action  of  wind  in  lifting  up  the  finer 
particles  of  disintegrated  rock  on  the  surface  of  the  land. 
Volcanic  explosions  sometimes  supply  prodigious  quantities 
of  fine  dust.  There  is  probably  also  some  addition  to  the 
solid  particles  in  the  atmosphere  from  the  explosion  and  dis- 
sipation of  meteorites  on  entering  our  atmosphere.  To  the 
wide  diffusion  of  minute  solid  particles  in  the  air  great  im- 
portance in  the  condensation  of  vapor  has  recently  been 
assigned.  (Book  III.  Part  II.  Section  ii.) 

The  comparatively  small,  but  by  no  means  unimportant, 
proportions  of  these  minor  components  of  the  atmosphere 
are  much  more  liable  to  variation  than  those  of  the  more 
essential  gases.  Chloride  of  sodium,  for  instance,  is,  as 
might  be  expected,  particularly  abundant  in  the  air  bor- 
dering the  sea.  Nitric  acid,  ammonia,  and  sulphuric  acid 

*  A  cubic  metre  of  air  at  the  freezing-point  can  hold  only  4-871  grammes  of 
water- vapor,  but  at  40°  C.  can  take  up  50-70  grammes.  One  cubic  mile  of  air 
saturated  with  vapor  at  35°  C.  will,  if  cooled  to  0°,  deposit  upward  of  140,000 
tons  of  water  as  rain.  Eoscoe  and  Schorlemmer's  "Chemistry,"  L  p.  452. 

4  See  Tyndall's  researches  which  established  this  important  function  of  the 
aqueous  vapor. of  the  atmosphere,  and  their  confirmation  by  meteorological 
observation.  S.  A.  Hill,  Proc.  Roy.  Soc.  xxxiii.  216,  435. 


66  TEXT-BOOK   OF   GEOLOGY 

appear  most  conspicuously  in  the  air  of  towns.  The  or- 
ganic substances  present  in  the  air  are  sometimes  living 
germs,  such  as  probably  often  lead  to  the  propagation  of 
disease,  and  sometimes  mere  fine  particles  of  dust  derived 
from  the  bodies  of  living  or  dead  organisms.6 

As  a  geological  agent,  the  atmosphere  effects  changes  by 
the  chemical  reactions  of  its  constituent  gases  and  vapors, 
by  its  varying  temperature,  and  by  its  motions.  Its  func- 
tions in  these  respects  are  described  in  Book  III.  Part.  11. 
Section  i. 

2.  The  Oceans. — Bather  less  than  three-fourths  of  the 
surface  of  the  globe  (or  about  144,712,000  square  miles)  are 
covered  by  the  irregular  sheet  of  water  known  as  the  Sea. 
Within  the  last  twenty  years,  much  new  light  has  been 
thrown  upon  the  depths,  temperatures,  and  biological  con- 
ditions of  the  ocean-basins,  more  particularly  by  the  "Light- 
ning," "Porcupine,"  "Challenger,"  "Tuscarora,"  "Blake," 
"Gazelle"  and  other  expeditions  fitted  out  by  the  British, 
American,  German  and  Norwegian  Governments."  It  has 
been  ascertained  that  few  parts  of  the  Atlantic  Ocean  ex- 
ceed 3000  fathoms,  the  deepest  sounding  obtained  there 
being  one  taken  about  100  miles  north  from  the  island  of 
St.  Thomas,  which  gave  3875  fathoms,  or  rather  less  than 
44  miles.  The  Atlantic  appears  to  have  an  average  depth 

B  The  air  of  towns  is  peculiarly  rich  in  impurities,  especially  in  manufactur- 
ing districts,  where  much  coal  is  used.  These  impurities,  however,  though  of 
serious  consequence  to  the  towns  in  a  sanitary  point  of  view,  do  not  sensibly 
affect  the  general  atmosphere,  seeing  that  they  are  probably  in  great  measure 
taken  out  of  the  air  by  rain,  even  in  the  districts  which  produce  them.  They 
possess,  nevertheless,  a  special  geological  significance,  and  in  this  respect,  too, 
have  important  economic  bearings.  See  on  this  whole  subject,  Angus  Smith's 
"Air  and  Rain,"  and  the  account  of  Rain  in  Book  III.  Part  II.  Sect.  ii. 

6  See  Wyville  Thomson,  "The  Depths  of  the  Sea,"  1873;  "The  Atlantic," 
1877;  "Report  of  'Challenger'  Expedition,"  especially  the  forthcoming  volumes 
giving  a  summary  of  results;  A.  Agassiz,  "Three  Cruises  of  the  'Blake,'  "  1888; 
"Den  Norske  Nordhavs-Expedition,"  1876-78. 


GEOGNOSY  67 

in  its  more  open  parts  of  from  2000  to  3000  fathoms,  or  from 
about  2  to  3\  miles.  In  the  Pacific  Ocean  H.M.  Ship  "Chal- 
lenger" got  soundings  of  3950  and  4475  fathoms,  or  about 
4\  and  rather  more  than  5  miles.  Since  then  the  TJ.  S.  Ship 
"Tuscarora"  obtained  a  still  deeper  sounding  (4655  fathoms), 
to  the  east  of  the  Kurile  Islands.  This  is  the  deepest  abyss 
yet  found  in  any  part  of  the  ocean.  But  these  appear  to 
mark  exceptionally  abysmal  depressions,  the  average  depth 
being,  as  in  the  Atlantic,  between  2000  and  3000  fathoms. 
We  may  therefore  assume,  as  probably  not  far  from  the 
truth,  that  the  average  depth  of  the  sea  is  about  2500  fath- 
oms, or  nearly  3  miles.  Its  total  cubic  contents  will  thus 
be  about  400  millions  of  cubic  miles. 

With  regard  also  to  the  form  of  the  bottom  of  the  great 
oceans,  much  additional  information  has  recently  been  ob- 
tained. Over  vast  areas  in  the  central  regions,  the  sea-floor 
appears  to  form  great  plains,  with  comparatively  few  in- 
equalities, but  with  lines  of  submarine  ridges,  comparable  to 
chains  of  hills  or  mountains  on  the  land.  Recent  sound- 
ings, however,  taken  at  short  distances,  have  revealed,  in 
parts  of  the  Atlantic  that  were  supposed  to  be  deep  and 
with  a  tolerably  uniform  bottom,  submarine  peaks  rising  to 
within  50  fathoms  from  the  surface.7  A  vast  central  ridge 
has  also  been  traced  down  the  length  of  this  ocean,  from 
which  a  few  lonely  peaks  rise  above  sea-level — the  Azores, 
St.  Paul,  Ascension,  and  Tristan  d'Acunha.  In  the  Pacific 
Ocean,  the  lines  of  coral-islands  appear  to  rise  on  subma- 
rine ridges,  having  a  general  northwesterly  and  southeast- 
erly trend.  It  is  significant  that  the  islands  which  thus 
appear  far  from  any  large  mass  of  land  are  either  coral-reefs 

1  "Times,"  7th  Dec.  1883.     [J.  Y.  Buchanan.] 


TEXT-BOOK   OF  GEOLOGY 

or  of  volcanic  origin,  and  contain  none  of  the  granites, 
schists  and  other  ordinary  continental  rocks.  St.  Helena 
and  Ascension  in  the  Atlantic,  and  the  Friendly  and  Sand- 
wich Islands  in  the  Pacific  Ocean  are  conspicuous  examples. 

Another  important  result  of  recent  deep-sea  research  is 
the  determination  of  the  relation  of  mediterranean  seas  to 
the  main  ocean.  These  basins,  such  as  the  North,  Mediter- 
ranean, and  Black  Seas,  the  Gulf  of  Mexico,  Caribbean  Sea, 
Baffin's  Bay,  Hudson's  Bay,  Sea  of  Okhotsk,  and  Chinese 
Sea,  belong  rather  to  the  continental  than  the  oceanic  areas 
of  the  earth's  surface.  An  elevation  of  a  few  hundred  fathoms 
would  convert  most  of  them  into  land,  with  here  and  there 
deep  water-filled  basins. 

A  question  of  high  importance  in  geological  inquiry  is 
the  form  of  the  surface  of  the  sea  or  what  is  usually  called 
the  sea-level.  It  has  been  generally  assumed  that  this  sur- 
face is  stable  and  uniform  and  nearly  that  of  an  ellipsoid  of 
revolution,  owing  its  equilibrium  to  the  force  of  gravity  on 
the  one  hand  and  the  centrifugal  force  of  rotation  on  the 
other.  But  in  recent  years  this  conception  has  been  called 
in  question  both  by  physicists  and  geologists.  Observations 
as  well  as  calculations  haye  shown  that  the  attraction  exer- 
cised by  masses  of  land  raises  the  level  of  the  adjacent  sea, 
and  attempts  have  been  made  to  determine  how  far  the  de- 
formation thus  caused  departs  from  the  mean  of  the  theoret- 
ical ellipsoid  of  revolution.  According  to  Bruns,  a  conti- 
nent may  cause  a  difference  of  more  than  3000  feet  between 
the  actual  level  of  the  sea  and  that  of  the  ellipsoid.  But 
the  results  of  such  calculations  will  greatly  depend  on  the 
assumption  on  which  they  start  as  to  the  nature  of  the 
earth's  crust.  R.  S.  Woodward  has  calculated  that  if 
the  continent  of  Europe  and  Asia  be  supposed  to  be  sim- 


GEOGNOSY  69 

ply  a  superficial  aggregation  of  matter  with  a  density  as 
great  as  the  parts  under  the  sea,  the  elevation  of  sea-level 
at  the  centre  of  the  continent  due  to  attraction  would  amount 
to  about  2900  feet,  but  that,  if  the  continental  mass  be  as- 
sumed to  imply  a  defect  of  density  underneath  it,  the  eleva- 
tion of  the  sea  at  the  centre  of  the  continent  due  to  attrac- 
tion would  be  only  about  10  feet.8  This  subject  is  further 
considered  in  Book  III.  Part  I.  Section  iii. 

The  water  of  the  ocean  is  distinguished  from  ordinary 
terrestrial  waters  by  a  higher  specific  gravity,  and  the  pres- 
ence of  so  large  a  proportion  of  saline  ingredients  as  to 
impart  a  strongly  salt  taste.  The  average  density  of  sea- 
water  is  about  1*026,  but  it  varies  slightly  in  different  parts 
even  of  the  same  ocean.  According  to  the  observations  of 
J.  Y.  Buchanan  during  the  "Challenger"  expedition,  some 
of  the  heaviest  sea-water  occurs  in  the  pathway  of  the  trade- 
winds  of  the  North  Atlantic,  where  evaporation  must  be 
comparatively  rapid,  a  density  of  1 '02781  being  registered. 
Where,  however,  large  rivers  enter  the  sea,  or  where  there 
is  much  melting  ice,  the  density  diminishes;  Buchanan 
found  among  the  broken  ice  of  the  Antarctic  Ocean  that  it 
had  sunk  to  1  -02418. •  A  series  of  soundings  taken  during 
the  "Vega"  expedition  in  the  Kara  Sea  (lat.  76°  18',  long. 
95°  30'  E.)  gave  a  progressive  increase  of  salinity  from  1-1 
at  the  surface  to  3 '4  at  30  fathoms,  the  surface  being  fresh- 
ened by  the  water  poured  into  the  sea  by  the  Siberian 
rivers. I0 

The   greater   density   of   sea-water   depends,    of   course, 

8  Bruns,  "Die  Fignr  der  Erde,"  Berlin,  1876;  R.  S.  Woodward,  Bull.  U.  S. 
Geol.  Surv.  No.  48,  p.  85  (1888). 

*  Buchanan,  Proc.  Roy.  Soc.  (1876),  vol.  xxiv. 

10  0.  PeUersson,  "Vega-Expeditionena  Yetenskapliga  lakttagelser, "  vol.  ii 
Stockholm,  1883. 


TO  TEXT-BOOK   OF   GEOLOGY 

upon  the  salts  which  it  contains  in  solution.  At  an  early 
period  in  the  earth's  history,  the  water  now  forming  the 
ocean,  together  with  the  rivers,  lakes  and  snowfields  of 
the  land,  existed  as  vapor,  in  which  were  mingled  many 
other  gases  and  vapors,  the  whole  forming  a  vast  atmosphere 
surrounding  the  still  intensely  hot  globe.  Under  the  enor- 
mous pressure  of  the  primeval  atmosphere,  the  first  con- 
densed water  might  have  had  a  temperature  little  below  the 
critical  one.11  In  condensing,  it  would  carry  down  with  it 
many  substances  in  solution.  The  salts  now  present  in 
sea- water  are  to  be  regarded  as  principally  derived  from 
the  primeval  constitution  of  the  sea,  and  thus  we  may  infer 
that  the  sea  has  always  been  salt.  It  is  probable,  however, 
that,  as  in  the  case  of  the  atmosphere,  the  composition  of 
the  ocean-water  has  acquired  its  present  character  only  after 
many  ages  of  slow  change,  and  the  abstraction  of  much 
mineral  matter  originally  contained  in  it.  There  is  evi- 
dence, indeed,  among  the  geological  formations  that  large 
quantities  of  lime,  silica,  chlorides  and  sulphates  have  in 
the  course  of  time  been  removed  from  the  sea.11 

But  it  is  manifest  also  that,  whatever  may  have  been 
the  original  composition  of  the  oceans,  they  have  for  a  vast 
section  of  geological  time  been  constantly  receiving  mineral 
matter  in  solution  from  the  land.  Every  spring,  brook  and 
river  removes  various  salts  from  the  rocks  over  which  it 
moves,  and  these  substances,  thus  dissolved,  eventually 
find  their  way  into  the  sea.  Consequently  sea-water  ought 
to  contain  more  or  less  traceable  proportions  of  every 

"  Q.  J.  Geol.  Soc.  xxxvi.  (1880),  pp.  112,  117.  Fisher,  "Physics  of  Earth's 
Crust,"  2d  edit.  p.  148. 

>*  Sterry  Hunt  supposed  that  the  saline  waters  of  North  America  derive  their 
mineral  ingredients  from  the  sediments  and  precipitates  of  the  sea  in  which  the 
Palaeozoic  rocks  were  deposited.  "Geological  and  Chemical  Essays,"  p.  104. 


GEOGNOSY  71 

substance  which  the  terrestrial  waters  can  remove  from 
the  land — in  short,  of  probably  every  element  present  in 
the  outer  shell  of  the  globe,  for  there  seems  to  be  no  con- 
stituent of  the  earth  which  may  not,  under  certain  circum- 
stances, be  held  in  solution  in  water.  Moreover,  unless 
there  be  some  counteracting  process  to  remove  these  mineral 
ingredients,  the  ocean-water  ought  to  be  growing,  insensibly 
perhaps,  salter,  for  the  supply  of  saline  matter  from  the 
land  is  incessant.  It  has  been  ascertained  indeed,  with 
some  approach  to  certainty,  that  the  salinity  of  the  Baltic 
and  Mediterranean  is  gradually  increasing." 

The  average  proportion  of  saline  constituents  in  the 
water  of  the  great  oceans  far  from  land  is  about  three  and 
a  half  parts  in  every  hundred  of  water.14  But  in  inclosed 
seas,  receiving  much  fresh  water,  it  is  greatly  reduced, 
while  in  those  where  evaporation  predominates  it  is  corre- 
spondingly augmented.  Thus  the  Baltic  water  contains 
from  one-seventh  to  nearly  a  half  of  the  ordinary  propor- 
tion in  ocean-water,  while  the  Mediterranean  contains  some- 
times one-sixth  more  than  that  proportion.  Forchhammer 
has  shown  the  presence  of  the  following  twenty-seven  ele- 
ments in  sea- water:  oxygen,  hydrogen,  chlorine,  bromine, 
iodine,  fluorine,  sulphur,  phosphorus,  nitrogen,  carbon, 


18  Paul,  in  Watts's  "Dictionary  of  Chemistry,"  v.  p.  1020.  For  a  detailed 
study  of  the  Eastern  Mediterranean,  see  the  Reports  of  a  Commission,  Denksch. 
Akad.  Wiss.  Vienna,  1892  et  seq. 

14  Dittmar's  elaborate  researches  on  the  samples  of  ocean  water  collected  by 
the  "Challenger"  expedition  show  that  the  lowest  percentage  of  salts  obtained 
was  3-301,  from  the  southern  part  of  the  Indian  Ocean,  south  of  lat.  66°,  while 
the  highest  was  3 -737,  from  the  middle  of  the  North  Atlantic,  at  about  lat.  23°. 
Some  valuable  results  from  observations  on  the  waters  of  the  North  Atlantic  are 
given  by  H.  Tornoe  and  L.  Schmelck  in  the  Report  of  the  Norwegian  North- 
Atlantic  Expedition,  1876-78.  The  average  proportion  of  salts  was  found  to 
be  from  3-47  to  3*51  per  cent,  the  mean  quantities  of  each  constituent  as  esti- 
mated being  as  follows:  CaCO,,  0-002;  CaSo4,  0-1395;  MgS04)  0-2071;  MgClt, 
0-3561;  KC1,  0-0747;  NaHC08,  0-0166;  NaCl,  2-682. 


72 


TEXT-BOOK   OF   GEOLOGY 


silicon,  boron,  silver,  copper,  lead,  zinc,  cobalt,  nickel, 
iron,  manganese,  aluminium,  magnesium,  calcium,  stron- 
tium, barium,  sodium,  and  potassium.15  To  these  may  be 
added  arsenic,  lithium,  caesium,  rubidium,  goldj  and  prob- 
ably most  if  not  all  of  the  other  elements,  though  in  pro- 
portions too  minute  for  detection.  The  chief  constituents 
have  been  determined  by  Dittmar  to  be  present  in  the  pro- 
portions shown  in  the  first  column  of  the  subjoined  tables. 
Assuming  them  to  occur  in  the  combinations  shown  in  the 
second  column,  they  are  present  in  the  average  ratios 
therein  stated:14 


Chlorine 56-292 

Bromine 0-188 

Sulphuric  acid,  SO3 6-410 

Carbonic  acid,  C0r 0-152 

Lime,  CaO 1-676 

Magnesia,  MgO 6-209 

Potash,  KO 1-332 

Na,0 41-234 


Subtract  Basic  Oxygen  equiv-  )    ,  0 .  .Q, 

alent  to  the  Halogens  [    l 

Total  Salts 100-000 


II 

Chloride  of  sodium 77-768 

Chloride  of  magnesium. 10-887 

Sulphate  of  magnesia 4-737 

Sulphate  of  lime. 3-600 

Sulphate  of  potash 2-466 

Bromide  of  magnesium. •  0-217 

Carbonate  of  lime 0-346 

Total  Salts 100-000 


Sea-water  is  appreciably  alkaline,  its  alkalinity  being  due 
to  the  presence  of  carbonates,  of  which  carbonate  of  lime  is 
one.17  In  addition  to  its  salts  it  always  contains  dissolved 


ls  Forchhammer,  Phil.  Trans,  civ.  p.  205.  According  to  Thorpe  and  Morton 
(Chem.  Soc.  Journ.  xxiv.  p.  507),  the  water  of  the  Irish  Sea  contains  in  summer 
rather  more  salts  than  in  winter.  In  1000  grammes  of  the  summer  water  of  the 
Irish  Sea  they  found  0-04754  grammes  of  carbonate  of  lime,  0-00503  of  ferrous 
carbonate  and  traces  of  silicic  acid.  For  exhaustive  chemical  investigations  re- 
garding the  chemistry  of  ocean-water  consult  Dittmar,  in  vol.  i.  "Physics  and 
Chemistry,"  Report  of  Voyage  of  the  "Challenger,"  1884;  also  the  "Chem- 
istry" part  of  the  Report  of  the  Norwegian  North-Atlantic  Expedition,  1876- 
1878. 

16  Dittmar,  op.  cit.  p.  203  et  seq.     For  further  reference  to  the  chemistry  of 
sea-water,  especially  in  connection  with  the  action  of  marine  organisms,  see 
posted,  p.  484. 

17  Dittmar,  op.  cit.  p.  200 


GEOGNOSY  73 

atmospheric  gases.  From  the  researches  conducted  during 
the  voyage  of  the  "Bonite'"  in  the  Atlantic  and  Indian 
Oceans,  it  was  estimated  that  the  gases  in  100  volumes  of 
sea- water  ranged  from  1-86  to  3-04,  or  from  two  to  three 
per  cent.  From  observations  made  during  the  "Porcupine" 
cruise  of  1868,  it  was  ascertained  that  the  proportion  of 
oxygen  was  greatest  in  the  surface  water,  and  least  in  the 
bottom  water.  The  dissolved  oxygen  and  nitrogen  are 
doubtless  absorbed  from  the  atmosphere,  the  proportion 
so  absorbed  being  mainly  regulated  by  temperature.  Ac- 
cording to  Dittmar's  recent  determinations,  a  litre  of  sea- 
water  at  0°  C.  will  take  up  15-60  cubic  centimetres  of  nitro- 
gen and  8-18  of  oxygen,  while  at  30°  C.  the  proportions 
sink  respectively  to  8'36  and  4-17.  He  regards  the  carbonic 
acid  as  occurring  chiefly  as  carbonates,  its  presence  in  the 
free  state  being  exceptional.  During  the  voyage  of  the 
"Challenger,"  Buchanan  ascertained  that  the  proportion 
of  carbonic  acid  is  always  nearly  the  same  for  similar  tem- 
peratures, the  amount  in  the  Atlantic  surface  water,  between 
20°  and  25°  C.,  being  0-0466  gramme  per  litre,  and  in  the 
surface  Pacific  water  0*0268;  and  that  sea-water  contains 
sometimes  at  least  thirty  times  as  much  carbonic  acid  as 
an  equal  bulk  of  fresh  water  would  do.18  A  supposed 
greater  proportion  of  carbonic  acid  in  the  deeper  and 
colder  waters  of  the  ocean  has  been  suggested  as  the  majn 
cause  of  the  disappearance  of  the  larger  and  more  delicate 
calcareous  pelagic  organisms  from  abysmal  deposits,  these 
forms  being  more  readily  attacked  and  carried  away  in 

18  Proc.  Roy.  Soc.  xxiv.  According  to  Mr.  Tornoe  (Norwegian  North- Atlantic 
Expedition,  1876-78,  "Chemistry")  most  of  the  carbonic  acid  of  sea-water  is  in 
combination  with  soda  as  bicarbonate  of  soda.  See  his  memoir  for  an  estimate 
of  the  proportion  of  air  in  sea-water;  also  J.  T.  Buchanan,  Nature,  xxv.  p.  386. 
Dittmar,  op.  cit.  p.  209. 

GEOLOGY— Vol.  XXIX— 4 


74  TEXT-BOOK   OF   GEOLOGY 

solution;  but  according  to  Dittmar,  even  alkaline  sea- 
water,  if  given  sufficient  time,  will  take  up  carbonate  of 
lime  in  addition  to  what  it  already  contains."  Another 
of  the  constituents  of  sea-water  is  diffused  organic  matter, 
derived  from  the  bodies  of  dead  plants  and  animals,  and  no 
doubt  of  great  importance  as  furnishing  food  for  the  lower 
grades  of  animal  life.*0 

II.— The  Solid  Globe  or  Lithosphere 

"Within  the  atmospheric  and  oceanic  envelopes  lies  the 
inner  solid  globe.  The  only  portion  of  it  which,  rising 
above  the  sea,  is  visible  to  us,  and  forms  what  we  term 
Land,  occupies  rather  more  than  one-fourth  of  the  total 
superficies  of  the  globe,  or  about  52,000,000  square  miles- 

§  1.  The  Outer  Surface.— The  land  is  placed  chiefly  in 
the  northern  hemisphere  and  is  disposed  in  large  masses, 
or  continents,  which  taper  southward  to  about  half  the  dis- 
tance between  the  equator  and  the  south  pole.  No  adequate 
cause  has  yet  been  assigned  for  the  present  distribution  of 
the  land.  It  can  be  shown,  however,  that  portions  of  the 
continents  are  of  extreme  geological  antiquity.  There  is 
reason  to  believe,  indeed,  that  the  present  terrestrial  areas 
have  on  the  whole  been  land,  or  have,  at  least,  never  been 
submerged  beneath  deep  water,  from  the  time  of  the  earliest 
stratified  formations;  and  that,  on  the  other  hand,  the 
ocean-basins  have  always  been  vast  areas  of  depression. 
This  subject  will  be  discussed  in  subsequent  pages. 

In  the  New  World,  the  continental   trend  is  approxi- 

19  Dittmar,  op.  cit.  p.  222. 

90  Different  estimates  have  been  made  of  the  proportion  of  organic  matter. 
According  to  the  researches  of  L.  Schmelck  (Norwegian  North-Atlantic  Expedi- 
tion, 1876-78,  Part.  ix.  p.  4),  the  proportion  is  0-0025  gramme  in  100  c.c.  of 


GEOGNOSY  75 

mately  north  and  south;  in  the  Old  World,  though  less 
distinctly  marked,  it  ranges  on  the  whole  east  and  west. 
The  intimate  relation  which  may  be  observed  between  this 
general  trend  and  the  direction  of  mountain  chains,  is  best 
exhibited  by  the  American  continent.  Europe  and  Africa 
may  be  considered  as  forming,  with  Asia,  the  vast  conti- 
nental mass  of  the  Old  World.  The  existing  severance  of 
Africa  and  Europe  is  of  comparatively  recent  date.  On  the 
other  hand,  Europe  and  Asia  were  not  always  so  continuous 
as  at  present.  But  even  where  the  continents  of  the  Old 
World  are  separated  by  sea,  the  intervening  hollows,  though 
now  covered  by  ocean-water,  must  be  regarded  as  essentially 
part  of  the  continental  areas.  Asia  is  linked  with  Australia 
by  a  chain  of  islands.  The  great  contrast  between  the  Asi- 
atic and  Australian  faunas,  however,  affords  good  grounds 
for  the  belief  that,  at  least  for  an  enormous  period  of  time, 
Asia  and  Australia  have  been  divided  by  an  important 
barrier  of  sea. 

While  any  good  map  of  the  globe  enables  us  to  see  at  a 
glance  the  relative  positions  and  areas  of  the  continents  and 
oceans,  most  maps  fail  to  furnish  any  data  by  which  the 
general  height  or  volume  of  a  continent  may  be  estimated. 
As  a  rule,  the  mountain-chains  are  exaggerated  in  breadth, 
and  incorrectly  indicated,  while  no  attempt  is  made  to  dis- 
tinguish between  high  plateaus  and  low  plains.  In  North 
America,  for  example,  a  continuous  shaded  ridge  is  placed 
down  the  axis  of  the  continent,  and  marked  "Kooky  Moun- 
tains," while  the  vast  level  or  gently  rolling  prairies  are  left 
with  no  mark  to  distinguish  them  from  the  maritime  plains 
of  the  Eastern  and  Southern  States.  In  reality  there  is  no 
such  continuous  mountain -chain.  The  so-called  "Rocky 
Mountains"  consist  of  many  independent  and  sometimes 


76 


TEXT-BOOK    OF   GEOLOGY 


widely  separated  ridges,  having  a  general  meridional  trend, 
and  rising  above  a  vast  plateau,  which  is  itself  4000  or  5000 
feet  in  elevation.  It  is  not  these  intermittent  ridges  which 
really  form  the  great  mass  of  the  land  in  that  region,  but  the 
widely  extended  lofty  plateau,  or  rather  succession  of  pla- 
teaus, which  supports  them.  In  Europe,  also,  the  Alps 
form  but  a  subordinate  part  of  the  total  bulk  of  the  land. 
If  their  materials  could  be  spread  out  over  the  continent,  it 
has  been  calculated  that  they  would  not  increase  its  height 
more  than  about  twenty-one  feet." 

Attempts  have  been  made  to  estimate  the  probable  aver- 
age height  which  would  be  attained  if  the  various  inequali- 
ties of  the  land  could  be  levelled  down.  Humboldt  esti- 
mated the  mean  height  of  Europe  to  be  about  671,  of  Asia 
1132,  of  North  America  748,  and  of  South  America  1151 
feet.88  Herschel  supposed  the  mean  height  of  Africa  to  be 
1800  feet."  These  figures,  though  based  on  the  best  data 
available  at  the  time,  are  no  doubt  much  under  the  truth. 
In  particular,  the  average  height  assigned  to  North  Amer- 
ica is  evidently  far  less  than  it  should  be;  for  the  great 
plains  west  of  the  Mississippi  Valley  reach  an  altitude  of 
about  5000  feet,  and  serve  as  the  platform  from  which  the 
mountain  ranges  rise.  The  height  of  Asia  also  is  obviously 


SI  M.  De  Lapparent  ("Trait*  de  G^ologie, "  3d  edit.  p.  57)  gives  the  follow- 
ing estimate  of  relative  heights  and  areas,  the  area  below  sea-level  being  taken 
as  0-6  of  the  whole. 
Zone    I.  (from  sea-level  to    200  metres)  covers  34-7  %  of  the  terrestrial  surface 


II. 
III. 
IV. 

V. 
VI. 
VII. 


200 

500  ' 
1000  ' 
2000  ' 
3000  ' 


500 
1000 


3000 
4000 


above   4000 


"Aise  Centrale,"  torn.  i.  p.  168. 


21-6 
21-4 
14-2 
3-7 
2-1 
1-7 


99-4  " 
»3  "physical  Geography,"  p.  119. 


GEOGNOSY  77 

much  greater  than  this  old  estimate.  Gr.  Leipoldt  has  com- 
puted the  mean  height  of  Europe  to  be  296-838  metres 
(973-628  feet).84  Prof.  A.  De  Lapparent  makes  the  mean 
height  of  the  land  of  the  globe  2120  feet,  and  estimates  the 
mean  height  of  Europe  to  be  958  feet,  Asia  2884,  Africa 
1975,  North  America  1952,  and  South  America  1762."  Dr. 
John  Murray  computes  these  heights  as  follows:  Europe 
939,  Asia  3189,  Africa  2021,  North  America  1888,  South 
America  2078,  Australia  805  feet,  general  mean  height  of 
land  2252  feet.88  It  is  of  some  consequence  to  obtain  as 
near  an  approximation  to  the  truth  in  this  matter  as  may 
be  possible,  in  order  to  furnish  a  means  of  comparison  be- 
tween the  relative  bulk  of  different  continents,  and  the 
amount  of  material  on  which  geological  changes  can  be 
effected. 

The  highest  elevation  of  the  surface  of  the  land  is  the 
summit  of  Mount  Everest,  in  the  Himalaya  range  (29,000 
feet);  the  deepest  depression  not  covered  by  water  is  that  of 
the  shores  of  the  Dead  Sea  (1300  feet  below  sea-level). 
There  are,  however,  many  subaqueous  portions  of  the  land 
which  sink  to  greater  depths.  The  bottom  of  the  Caspian 
Sea,  for  instance,  lies  about  3000  feet  below  the  general  sea- 
level.  The  vertical  difference  between  the  highest  point 
of  the  land  and  the  maximum  known  depth  of  the  sea  is 
56,932  feet  or  nearly  11  miles. 

There  are  two  conspicuous  junction- lines  of  the  land 
with  its  overlying  and  surrounding  envelopes.  First,  with 

24  "Die  MittlereHoheEuropas,"  Leipzig,  1874.    In  this  work  the  mean  height 
of  Switzerland  is  put  down  as  1299-91  metres;  Spanish  peninsula,  700-60;  Aus- 
tria, 517-87;  Italy,  517-17;  Scandinavia,  428-10;  Prance,  393 -84;  Great  Britain, 
217-70;  German  Empire,   213-66;  Russia,   167-09;  Belgium,  163-36;  Denmark 
(exclusive  of  Iceland),  35-20;  the  Netherlands  (exclusive  of  Luxemburg  and  the 
tracts  below  sea-level),  9 '61. 

25  "TraLte","  p.  56.  26  Scottish  Geog.  Mag.  iv.  (1888),  23. 


78  TEXT-BOOK  OF    GEOLOGY 

the  Air,  expressed  by  the  contours  or  relief  of  the  land. 
Second,  with  the  Sea,  expressed  by  coast-lines. 

(1.)  Con  tours  or  Belief  of  the  Land. — While 
the  surface  of  the  land  presents  endless  diversities  of  de- 
tail, its  leading  features  may  be  generalized  as  mountains, 
table-lands,  and  plains. 

Mountains. — The  word  "mountain"  is,  properly  speak- 
ing, not  a  scientific  term.  It  includes  many  forms  of  ground 
utterly  different  from  each  other  in  size,  shape,  structure, 
and  origin.  It  is  popularly  applied  to  any  considerable  emi- 
nence or  range  of  heights,  but  the  height  and  size  of  the  ele- 
vated ground  so  designated  vary  indefinitely.  In  a  really 
mountainous  country  the  word  would  be  restricted  to  the 
loftier  masses  of  ground,  while  such  a  word  as  hill  would 
be  given  to  the  lesser  heights.  But  in  a  region  of  low  or 
gently  undulating  land,  where  any  conspicuous  eminence 
becomes  important,  the  term  mountain  is  lavishly  used. 
In  Eastern  America  this  habit  has  been  indulged  in  to 
such  an  extent,  that  what  are,  so  to  speak,  mere  hum- 
mocks in  the  general  landscape,  are  dignified  by  the 
name  of  mountains. 

It  is  hardly  possible  to  give  a  precise  scientific  definition 
to  a  term  so  vaguely  employed  in  ordinary  language. 
When  a  geologist  uses  the  word,  he  must  either  be  con- 
tent to  take  it  in  its  familiar  vague  sense,  or  must  add  some 
phrase  defining  the  meaning  which  he  attaches  to  it.  He 
finds  that  there  are  three  leading  and  totally  distinct  types 
of  elevation  which  are  all  popularly  termed  mountains.  1. 
Single  eminences,  standing  alone  upon  a  plain  or  table-land. 
This  is  essentially  the  volcanic  type.  The  huge  cones  of 
Vesuvius,  Etna,  and  Teneriffe,  as  well  as  the  smaller  ones 
so  abundant  in  volcanic  districts,  are  examples  of  it.  There 


GEOGNOSY  79 

occur,  however,  occasional  isolated  eminences  that  stand  up 
as  remnants  of  once  extensive  rock-formations.  These  have 
no  real  analogy  with  volcanic  elevations,  but  should  be 
classed  under  the  next  type.  The  remarkable  buttes  of 
Western  America  are  good  illustrations  of  them.  2.  Groups 
of  eminences  connected  at  the  sides  or  base,  often  forming 
lines  of  ridge  between  divergent  valleys,  and  owing  their 
essential  forms  not  to  underground  structure  so  much  as  to 
superficial  erosion.  Many  of  the  more  ancient  uplands, 
both  in  the  Old  World  and  the  New,  furnish  examples  of 
this  type,  such  as  the  Highlands  of  Scotland,  the  hills  of 
Cumberland  and  Wales,  the  high  grounds  between  Bohemia 
and  Bavaria,  the  Laurentide  Mountains  of  Canada,  and  the 
Green  and  White  Mountains  of  New  England.  3.  Lines  of 
lofty  ridge  rising  into  a  succession  of  more  or  less  distinct 
summits,  their  general  external  form  having  relation  to  an 
internal  plication  of  their  component  rocks.  These  linear 
elevations,  whose  existence  and  trend  have  been  determined 
immediately  by  subterranean  movement,  are  the  true  moun- 
tain-ranges of  the  globe.  They  may  be  looked  upon  as  the 
crests  of  the  great  waves  into  which  the  crust  of  the  earth 
has  been  thrown.  All  the  great  mountain-lines  of  the  world 
belong  to  this  type. 

Leaving  the  details  of  mountain-form  to  be  described  in 
Book  VII.,  we  may  confine  our  attention  here  to  a  few  of 
the  more  important  general  features.  In  elevations  of  the 
third  or  true  mountain  type,  there  may  be  either  one  line  or 
range  of  heights,  or  a  series  of  parallel  and  often  coalescent 
ranges.  In  the  Western  Territories  of  the  United  States, 
the  vast  plateau  has  been,  as  it  were,  wrinkled  by  the  uprise 
of  long  intermittent  ridges,  with  broad  plains  and  basins  be- 
tween them.  Each  of  these  forms  an  independent  moun- 


80  TEXT-BOOK   OF   GEOLOGY 

tain-range.  In  the  heart  of  Europe,  the  Bernese  Oberland, 
the  Pennine,  Lepontine,  Ehaetic,  and  other  ranges  form  one 
great  Alpine  chain  or  system. 

In  a  great  mountain-chain,  such  as  the  Alps,  Himalayas, 
or  Andes,  there  is  one  general  persistent  trend  for  the  suc- 
cessive ridges.  Here  and  there,  lateral  offshoots  may  di- 
verge, but  the  dominant  direction  of  the  axis  of  the  main 
chain  is  generally  observed  by  its  component  ridges  until 
they  disappear.  Yet  while  the  general  parallelism  is  pre- 
served, no  single  range  may  be  traceable  for  more  than  a 
comparatively  short  distance;  it  may  be  found  to  pass  in- 
sensibly into  another,  while  a  third  may  be  seen  to  begin 
on  a  slightly  different  line,  and  to  continue  with  the  same 
dominant  trend  until  it  in  turn  becomes  confluent.  The 
various  ranges  are  thus  apt  to  assume  an  arrangement  en 
echelon. 

The  ranges  are  separated  by  longitudinal  valleys,  that  is, 
depressions  coincident  with  the  general  direction  of  the 
chain.  These,  though  sometimes  of  great  length,  are  rela- 
tively of  narrow  width.  The  valley  of  the  Ehone,  from  the 
source  of  the  river  down  to  Martigny,  offers  an  excellent 
example.  By  a  second  series  of  valleys  the  ranges  are 
trenched,  often  to  a  great  depth,  and  in  a  direction  transverse 
to  the  general  trend.  The  Ehone  furnishes  also  an  example 
of  one  of  these  transverse  valleys,  in  its  course  from  Mar- 
tigny to  the  Lake  of  Geneva.  In  most  mountain  regions, 
the  heads  of  two  adjacent  transverse  valleys  are  often  con- 
nected by  a  depression  or  pass  (col,  joch). 

A  large  block  of  mountain  ground,  rising  into  one  or 
more  dominant  summits,  and  more  or  less  distinctly  denned 
by  longitudinal  and  traverse  valleys,  is  termed  in  French  a 
massif — a  word  for  which  there  is  no  good  English  equiva- 


GEOGNOSY  81 

lent.     Thus  in  the  Swiss  Alps  we  have  the  massifs  of  the 
Grlarnisch,  the  Todi,  the  Matterhorn,  the  Jungfrau,  etc. 

Very  exaggerated  notions  are  common  regarding  the 
angle  of  declivity  in  mountains.  Sections  drawn  across 
any  mountain  or  mountain-chain  on  a  true  scale,  that  is, 
with  the  length  and  height  on  the  same  scale,  bring  out  the 
fact  that,  even  in  the  loftiest  mountains,  the  breadth  of  base 
is  always  very  much  greater  than  the  height.  Actual  ver- 
tical precipices  are  less  frequent  than  is  usually  supposed, 
and  even  when  they  do  occur,  generally  form  minor  inci- 
dents in  the  declivities  of  mountains.  Slopes  of  more  than 
80°  in  angle  are  likewise  far  less  abundant  than  casual  tour- 
ists believe.  Even  such  steep  declivities  as  those  of  38°  or 
40°  are  most  frequently  found  as  talus-slopea  at  the  foot  of 
crumbling  cliffs,  and  represent  the  angle  of  repose  of  the 
disintegrated  debris.  Here  and  there,  where  the  blocks 
loosened  by  weathering  are  of  large  size,  they  may  accumu- 
late upon  each  other  in  such  a  manner  that  for  short  dis- 
tances the  average  angle  of  declivity  may  mount  as  high  as 
65°.  But  such  steep  slopes  are  of  limited  extent.  Declivi- 
ties exceeding  40°,  and  bearing  a  large  proportion  to  the 
total  dimensions  of  hill  or  mountain,  are  always  found  to 
consist  of  naked  solid  rock.  In  estimating  angles  of  incli- 
nation from  a  distance,  the  student  will  learn  by  practice 
how  apt  is  the  eye  to  be  deceived  by  perspective  and  to  ex- 
aggerate the  true  declivity,  sometimes  to  mistake  a  horizon- 
tal for  a  highly  inclined  or  vertical  line.  The  mountain 
outline  shown  in  Fig.  2  presents  a  slope  of  25°  between  a 
and  5,  of  45°  between  b  and  c,  of  17°  between  c  and  c?,  of  40° 
between  d  and  c,  and  of  70°  between  e  and  /.  At  a  great 
distance,  or  with  bad  conditions  of  atmosphere,  these  might 
be  believed  to  be  the  real  declivities.  Yet  if  the  same  an- 


82  TEXT-BOOK    OF   GEOLOGY 

gles  be  observed  in  another  way  (as  on  a  cottage  roof  at  B), 
we  may  learn  that  an  apparently  inclined  surface  may  really 
^         be  horizontal  (as  from  a  to  b  and  from  c  to  c£),  and 
that  by  the  effect  of  perspective,  slopes  may  be 
made  to  appear  much  steeper  than  they  really  are." 
Much  evil  has  resulted  in  geological  research 
from  the  use  of  exaggerated  angles  of  slope  in 
sections  and  diagrams.     It  is  therefore  desirable 
that   the   student    should,    from    the    beginning, 
accustom  himself  to  the  drawing  of  outlines  as 


•§• 


\ 


Pig.  2.— Angles  of  Slope  where  the  eye  may  be  deceived  by  per- 
spective.   (After  Ruskin.)    A,  Mountain  outline;  B,  The  same 
outline  as  shown  by  cottage  roof 

nearly  as  possible  on  a  true  scale.  The  accom- 
panying section  of  the  Alps  by  De  la  Beche 
(Fig.  3)  is  of  interest  in  this  respect,  as  one  of  the 
earliest  illustrations  of  the  advantage  of  con- 
structing geological  sections  on  a  true  scale  as 
to  the  relative  proportions  of  height  and  length." 

87  Mr.  Ruskin  has  well  illustrated  this  point.  See  "Modern 
Painters,"  vol.  iv.  p.  183,  whence  the  illustrations  in  the  text 
are  taken. 

28  '  'Sections  and  Views,  illustrative  of  Geological  Phenomena," 
1830.  Geol.  Observer,  p.  646. 


GEOGNOSY  83 

Table-lands  or  Plateaus  are  elevated  regions  of  flat  or  un- 
dulating country,  rising  to  heights  of  1000  feet  and  upward 
above  the  level  of  the  sea.  They  are  sometimes  bordered 
with  steep  slopes,  which  descend  from  their  edges,  as  the 
table-land  of  the  Spanish  peninsula  does  into  the  sea.  In 
other  cases,  they  gradually  sink  into  the  plains  and  have  no 
definite  boundaries;  thus  the  prairie-land  west  of  the  Mis- 
souri slowly  and  imperceptibly  ascends  until  it  becomes  a 
vast  plateau  from  4000  to  5000  feet  above  the  sea.  Occa- 
sionally a  high  table-land  is  encircled  with  lofty  mountains, 
as  in  those  of  Quito  and  Titicaca  among  the  Andes,  and  that 
of  the  heart  of  Asia;  or  it  forms  in  itself  the  platform  on 
which  lines  of  mountains  stand,  as  in  North  America,  where 
the  ranges  included  within  the  Rocky  Mountains  reach  ele- 
vations of  from  10,000  to  14,000  feet  above  the  sea,  but  not 
more  than  from  5000  to  10,000  feet  above  the  table-land. 

Two  types    of    table-land   structure   may   be   observed. 

1.  Table-lands    consisting    of    level    or    gently    undulated 
sheets  of  rock,   the  general  surface  of  the  country  corre- 
sponding with  that  of  the  stratification.     The  Kocky  Moun- 
tain plateau  is  an  example  of  this  type,  which  may  be  called 
that  of  Deposit,  for  the  flat  strata  have  been  equably  up- 
raised nearly  in  the  position  in  which  they  were  deposited. 

2.  Table-lands  formed  out  of  contorted,  crystalline,  or  other 
rocks,  which  have  been  planed  down  by  superficial  agents. 
This  type,  where  the  external  form  is  independent  of  geo- 
logical  structure,    may   be  termed   that  of   Erosion.     The 
fjelds  of  Norway  are  portions  of  such   a   table-land.      In 
proportion  to  its  antiquity,  a  plateau  is  trenched  by  run- 
ning water  into  systems  of  valleys,  until  in  the  end  it  may 
lose  its  plateau  character  and  pass  into  the  second  type  of 
mountain-ground  above  described.     This  change  has  largely 


84  TEXT-BOOK    OF   GEOLOGY 

altered  the  ancient  table-land  of  Scandinavia,  as  will  be 
illustrated  in  Book  VII. 

Plains  are  tracts  of  lowland  (under  1000  feet  in  height) 
which  skirt  the  sea-board  of  the  continents  and  stretch  in- 
land up  the  river  valleys.  The  largest  plain  in  the  world 
is  that  which,  beginning  in  the  centre  of  the  British  Islands, 
stretches  across  Europe  and  Asia.  On  the  west,  it  is 
bounded  by  the  ancient  table-lands  of  Scandinavia,  Scot- 
land and  Wales  on  the  one  hand,  and  those  of  Spain,  France 
and  Germany  on  the  other.  Most  of  its  southern  boundary 
is  formed  by  the  vast  belt  of  high  ground  which  spreads 
from  Asia  Minor  to  the  east  of  Siberia.  Its  northern  mar- 
gin sinks  beneath  the  waters  of  the  Arctic  Ocean.  This 
vast  region  is  divided  into  an  eastern  and  western  tract  by 
the  low  chain  of  the  Ural  Mountains,  south  of  which  its 
general  level  sinks,  until  underneath  the  Caspian  Sea  it 
reaches  a  depression  of  about  3000  feet  below  sea-level. 
Along  the  eastern  sea-board  of  America  lies  a  broad  belt 
of  low  plains,  which  attain  their  greatest  dimensions  in  the 
regions  watered  by  the  larger  rivers.  Thus  they  cover 
thousands  of  square  miles  on  the  north  side  of  the  Gulf  of 
Mexico,  and  extend  for  hundreds  of  miles  up  the  valley  of 
the  Mississippi.  Almost  the  whole  of  the  valleys  of  the 
Orinoco,  Amazon  and  La  Plata  is  occupied  with  vast  plains. 

From  the  evidence  of  upraised  marine  shells,  it  is  certain 
that  large  portions  of  the  great  plain  of  the  Old  World  com- 
paratively recently  formed  part  of  the  sea-floor.  It  is  like- 
wise probable  that  the  beds  of  some  inclosed  sea-basins, 
such  as  that  of  the  North  Sea,  have  formerly  been  plains 
of  the  dry  land. 

It  is  obvious,  from  their  distribution  along  river- valleys, 
and  on  the  areas  between  the  base  of  high  grounds  and  the 


GEOGNOSY  85 

sea,  that  plains  are  essentially  areas  of  deposit.  They  are 
the  tracts  that  have  received  the  detritus  washed  down  from 
the  slopes  above  them,  whether  that  detritus  has  originally 
accumulated  on  the  land  or  below  the  sea.  Their  surface 
presents  everywhere  loose  sandy,  gravelly,  or  clayey  forma- 
tions, indicative  of  its  comparatively  recent  subjection  to 
the  operation  of  running  water. 

(2.)  Coast-lines. — A  mere  inspection  of  a  map  of  the 
globe  brings  before  the  mind  the  striking  differences  which 
the  masses  of  land  present  in  their  line  of  junction  with 
the  sea.  As  a  rule,  the  southern  continents  possess  a  more 
uniform  unindented  coast-line  than  the  northern.  It  has 
been  estimated  that  the  ratios  between  area  and  coast-line 
among  the  different  continents,  stand  approximately  as  in 
the  following  table: 

(  Europe  has  1  geographical  mile  of  coast-line  to  143  sq.  m.  of  surface 

Northern  \  North  America  '' 


(  Asia  including  the  islands 
Africa 


(  Africa 

•<  South  Americ 

(  Australia 


Southern  <  South  America 


265 
469 
895 
434 
332 


In  estimating  the  relative  potency  of  the  sea  and  of  the 
atmospheric  agents  of  disintegration,  in  the  task  of  wearing 
down  the  land,  it  is  evidently  of  great  importance  to  take 
into  account  the  amount  of  surface  respectively  exposed 
to  their  operations.  Other  things  being  equal,  there  is 
relatively  more  marine  erosion  in  Europe  than  in  North 
America.  But  we  require  also  to  consider  the  nature  of 
the  coast-line,  whether  flat  and  alluvial,  or  steep  and  rocky, 
or  with  some  intermediate  blending  of  these  two  characters. 
By  attending  to  this  point,  we  are  soon  led  to  observe  such 
great  differences  in  the  character  of  coast-lines,  and  such 
an  obvious  relation  to  differences  of  geological  structure, 
on  the  one  hand,  and  to  diversities  in  the  removal  or  de- 


86  TEXT-BOOK   OF   GEOLOGY 

posit  of  material,  on  the  other,  as  to  suggest  that  the  present 
coast-lines  of  the  globe  cannot  be  aboriginal,  but  must  be 
referred  to  the  operation  of  geological  agents  still  at  work. 
This  inference  is  amply  sustained  by  more  detailed  investi- 
gation. While  the  general  distribution  of  land  and  water 
must  undoubtedly  be  assigned  to  terrestrial  movements 
affecting  the  solid  globe,  the  present  actual  coasts  of  the 
land  have  chiefly  been  produced  by  local  causes.  Head- 
lands project  from  the  land  because,  for  the  most  part,  they 
consist  of  rock  which  has  been  better  able  to  withstand  the 
shock  of  the  breakers.  Bays  and  creeks,  on  the  other  hand, 
have  been  cut  by  the  waves  out  of  less  durable  materials. 
Again,  by  the  sinking  of  land,  ranges  of  hills  have  become 
capes  and  headlands,  while  the  valleys  have  passed  into 
the  condition  of  bays,  inlets,  or  fjords.  By  the  uprise  of 
the  sea-bottom,  tracts  of  low  alluvial  ground  have  been 
added  to  the  land.  Hence,  speculations  as  to  the  history 
of  the  elevation  of  the  land,  based  merely  upon  inferences 
from  the  form  of  .coast-lines  as  expressed  upon  ordinary 
maps,  to  be  of  real  service,  demand  a  careful  scrutiny  of 
the  actual  coast-lines,  and  an  amount  of  geological  investi- 
gation which  would  require  long  and  patient  toil  for  its 
accomplishment. 

Passing  from  the  mere  external  form  of  the  land  to  the 
composition  and  structure  of  its  materials,  we  may  begin 
by  considering  the  general  density  of  the  entire  globe,  com- 
puted from  observations  and  compared  with  that  of  the 
outer  and  accessible  portion  of  the  planet.  Eeference  has 
already  been  made  to  the  comparative  density  of  the  earth 
among  the  other  members  of  the  solar  system.  In  inquiries 
regarding  the  history  of  our  globe,  the  density  of  the  whole 


GEOGNOSY  87 

mass  of  the  planet,  as  compared  with  water — the  standard 
to  which  the  specific  gravities  of  terrestrial  bodies  are  re- 
ferred— is  a  question  of  prime  importance.  Various  methods 
have  been  employed  for  determining  the  earth's  density. 
The  deflection  of  the  plumb-line  on  either  side  of  a  moun- 
tain of  known  structure  and  density,  the  time  of  oscillation 
of  the  pendulum  at  great  heights,  at  the  sea-level,  and  in 
deep  mines,  and  the  comparative  force  of  gravitation  as 
measured  by  the  torsion  balance,  have  each  been  tried  with 
the  following  various  results: 

Plumb-line  experiments  on  Schichallien  (Maskelyne  and  Playfair)  gave 

as  the  mean  density  of  the  earth 4-713 

Do.  on  Arthur's  Seat,  Edinburgh  (James) 5-316 

Pendulum  experiments  on  Mont  Cenis  (Carlini  and  Giulio) 4-950 

Do.  in  Harton  coal-pit,  Newcastle  (Airy) 6'565 

Torsion  balance  experiments  (Cavendish,  1798) 5-480 

Do.  do.  (Reich,  1838) 5-49 

Do.  do.  (Baily,  1843) 5-660 

Do.  do.  (Corou  and  Bailie,  1872-73) 5-50-5-56 

Though  these  observations  are  somewhat  discrepant,  we 
may  feel  satisfied  that  the  globe  has  a  mean  density  neither 
much  more  nor  much  less  than  6 -5;  that  is  to  say,  it  is  five 
and  a  half  times  heavier  than  one  of  the  same  dimensions 
formed  of  pure  water.  Now  the  average  density  of  the 
materials  which  compose  the  accessible  portions  of  the 
earth  is  between  2 '5  and  3;  so  that  the  mean  density  of 
the  whole  globe  is  about  twice  as  much  as  that  of  its  outer 
part.  We  might,  therefore,  infer  that  the  inside  consists 
of  much  heavier  materials  than  the  outside,  and  conse- 
quently that  the  mass  of  the  planet  must  contain  at  least 
two  dissimilar  portions — an  exterior  lighter  crust  or  rind, 
and  an  interior  heavier  nucleus.  But  the  effect  of  pressure 
must  necessarily  increase  the  specific  gravity  of  the  interior, 
as  will  be  alluded  to  further  on. 

§  2.  The  Crust. — It  was  formerly  a  prevalent  belief  that 


88  TEXT-BOOK    OF   GEOLOGY 

the  exterior  and  interior  of  the  globe  differed  from  each 
other  to  such  an  extent  that,  while  the  outer  parts  were 
cool  and  solid,  the  vastly  more  enormous  inner  intensely 
hot  part  was  more  or  less  completely  liquid.  Hence  the 
term  "crust"  was  applied  to  the  external  rind  in  the  usual 
sense  of  that  word.  This  crust  was  variously  computed  to 
be  ten,  fifteen,  twenty,  or  more  miles  in  thickness.  In  the 
accompanying  diagram  (Fig.  4),  for  example,  the  thick  line 
forming  the  circle  represents  a  relative  thickness  of  100 
miles.  There  are  so  many  proofs  of  enormous  and  wide- 
spread corrugation  of  the 
materials  of  the  earth's 
outer  layers,  and  such 
abundant  traces  of  former 
volcanic  action,  that  geol- 
ogists have  naturally  re- 
garded the  doctrine  of  a 
thin  crust  over  a  liquid  in- 
terior as  necessary  for  the 
explanation  of  a  large 
class  of  terrestrial  phe- 

Fig.  4.— Supposed  Crust  of  the  Earth,  _, 

100  Miles  thick  nomena.         For      reasons 

which  will  be  afterward  given,  however,  this  doctrine 
has  been  opposed  by  eminent  physicists,  and  is  now  aban- 
doned by  most  geologists.  Nevertheless  the  term  "crust" 
continues  to  be  used,  apart  from  all  theory  regarding  the 
nucleus,  as  a  convenient  word  to  denote  those  cool,  upper 
or  outer  layers  of  the  earth's  mass  in  the  structure  and 
history  of  which,  as  the  only  portions  of  the  planet  accessi- 
ble to  human  observation,  lie  the  chief  materials  of  geolog- 
ical investigation.  The  chemical  and  mineral  constitution 
of  the  crust  is  fully  discussed  in  later  pages  (p.  112  et  seq.}. 


GEOGNOSY  89 

§  3.  The  Interior  or  Nucleus.— Though  the  mere  outside 
skin  of  our  planet  is  all  with  which  direct  acquaintance  can 
be  expected,  the  irregular  distribution  of  materials  beneath 
the  crust  may  be  inferred  from  the  present  distribution  of 
land  and  water,  and  the  observed  differences  in  the  amount 
of  deflection  of  the  plumb-line  near  the  sea  and  near  moun- 
tain-chains. The  fact  that  the  southern  hemisphere  is 
almost  wholly  covered  with  water,  appears  only  explica- 
ble, as  already  remarked,  on  the  assumption  of  an  excess 
of  density  in  the  mass  of  that  half  of  the  planet.  The 
existence  of  such  a  vast  sheet  of  water  as  that  of  the 
Pacific  Ocean  is  to  be  accounted  for,  says  Archdeacon 
Pratt,  by  the  presence  of  "some  excess  of  matter  in  the 
solid  parts  of  the  earth  between  the  Pacific  Ocean  and 
the  earth's  centre,  which  retains  the  water  in  its  place, 
otherwise  the  ocean  would  flow  away  to  the  other  parts  of 
the  earth."  "  The  same  writer  points  out  that  a  deflection 
of  the  plumb-line  toward  the  sea,  which  has  in  a  number 
of  cases  been  observed,  indicates  that  "the  density  of  the 
crust  beneath  the  mountains  must  be  less  than  that  below 
the  plains,  and  still  less  than  that  below  the  ocean-bed.34 
Apart,  therefore,  from  the  depressions  of  the  earth's  sur- 
face, in  which  the  oceans  lie,  we  must  regard  the  internal 
density,  whether  of  crust  or  nucleus,  to  be  somewhat  irreg- 
ularly arranged — there  being  an  excess  of  heavy  materials 
in  the  water- hemisphere,  and  beneath  the  ocean-beds  as 
compared  with  the  continental  masses. 

It  has  been  argued  from  the  difference  between  the 
specific  gravity  of  the  whole  globe  and  that  of  the  crust, 

99  "Figure  of  the  Earth,"  4th  edit.  p.  236. 

30  Op.  cit.  p.  200.  See  also  Herschel,  "Phys.  Geog."  §  13;  O.  Fisher, 
Cambridge  Phil.  Trans,  xii.  part  ii. ;  "Physics  of  the  Earth's  Crust,"  p.  75. 
Phil.  Mag.  July,  1886.  Faye,  Comptes  reudus,  cii.  (1886),  p.  651. 


TEXT-BOOK    OF   GEOLOGY 

that  the  interior  must  consist  of  heavier  material,  and  may 
be  metallic.  But  the  effect  of  the  enormous  internal  press- 
ure, it  might  be  supposed,  should  make  the  density  of  the 
nucleus  much  higher,  even  if  the  interior  consisted  of 
matter  which,  on  the  surface,  would  be  no  heavier  than 
that  of  the  crust.  In  fact,  we  might,  on  the  contrary,  argue 
for  the  probable  comparative  lightness  of  the  substance 
composing  the  nucleus.  That  the  total  density  of  the 
planet  does  not  greatly  exceed  its  observed  amount,  may 
indicate  that  some  antagonistic  force  counteracts  the  effect 
of  pressure.  The  only  force  we  can  suppose  capable  of  so 
acting  is  heat,  though  to  what  extent  this  counterbalancing 
takes  place  is  still  unknown.  It  must  be  admitted  that  we 
are  still  in  ignorance  of  the  law  that  regulates  the  com- 
pression of  solids  under  such  vast  pressure  as  must  exist 
within  the  earth's  interior.  We  know  that  gases  and  va- 
pors may  be  compressed  into  liquids,  sometimes  even  into 
solids,  and  that  in  the  liquid  condition  another  law  of  com- 
pressibility begins.  We  know  also  from  experiment  that 
some  substances  have  their  melting-point  raised  by  press- 
ure.81 It  may  be  that  the  same  effect  takes  place  within 
the  earth;  that  pressure  increasing  inward  to  the  centre  of 
the  globe,  while  augmenting  the  density  of  each  successive 
shell,  may  retain  the  whole  in  a  solid  condition,  yet  at 
temperatures  far  above  the  normal  melting-points  at  the 
surface.  Hence,  on  this  view  of  the  matter,  it  is  conceiv- 
able that  the  difference  between  the  density  of  the  whole 
globe  and  that  of  the  crust  may  be  due  to  pressure,  rather 
than  to  any  essential  difference  of  composition.  Laplace 
proposed  the  hypothesis  that  the  increase  of  the  square  of 

31  Under  a  pressure  of  792  atmospheres,  spermaceti  has  its  melting-point 
raised  from  51*  to  80 '2°,  and  wax  from  64 -5°  to  80 '2°. 


GEOGNOSY  91 

the  density  is  proportional  to  the  increase  of  the  pressure, 
which  gives  a  density  of  8-23  at  half  the  terrestrial  radius 
and  of  10 '74  at  the  centre.  From  another  law  proposed  by 
Prof.  Darwin,  the  density  at  half  the  radius  is  only  7-4, 
but  thence  toward  the  centre  increases  rapidly  up  to  infin- 
ity." Dr.  Pfaff  believes  that  the  mean  terrestrial  density 
of  5*5  is  not  incompatible  with  the  notion  that  the  whole 
globe  consists  of  materials  of  the  same  density  as  the  rocks 
of  the  crust."  It  is  possible  that  the  gases  dissolved  in  the 
hot  magma  of  the  nucleus,  with  their  very  high  tension, 
may  counteract  the  effects  of  compression  and  thus  reduce 
density. 

Analogies,  in  the  solar  system,  however,  as  well  as  the 
actual  structure  of  the  rocky  crust  of  the  globe,  suggest 
that  heavier  metallic  ingredients  possibly  predominate  in 
the  nucleus.  If  the  materials  of  the  globe  were  once,  as 
they  are  believed  to  have  been,  in  a  liquid  condition,  they 
would  then  doubtless  be  subject  to  internal  arrangement,  in 
accordance  with  their  relative  specific  gravities.  We  may 
conceive  that,  as  in  the  case  of  the  sun,  as  well  as  of  the 
solar  system  generally  (ante,  p.  25),  there  would  be,  so  long 
as  internal  mobility  lasted,  a  tendency  in  the  denser  ele- 
ments of  our  planet  to  gravitate  toward  the  centre,  in  the 
lighter  to  accumulate  outside.  That  a  distribution  of  this 
nature  has  certainly  taken  place  to  some  extent,  is  evident 
from  the  structure  of  the  envelopes  and  crust.  It  is  what 
might  be  expected,  if  the  constitution  of  the  globe  resem- 
bles, on  a  small  scale,  the  larger  planetary  system  of  which 


32  See  Fisher  "Physics  of  Earth's  Crust,"  2d  edit.  chap.  ii.     Legendre  sup- 
posed that  the  density  being  2-5  at  the  surface,  it  is  8-5  at  half  the  length  of  the 
radius  and  11-3  at  the  centre.     More  recently  E.  Roche  calculated  these  densi- 
ties to  be  2'1,  8-5  and  10-6  respectively. 

33  "Allgemeine  Geologie  als  exacte  Wissenschaf t, "  p.  42. 


92  TEXT-BOOK   OF   GEOLOGY 

it  forms  a  part.  The  existence  even  of  a  metallic  interior 
has  been  inferred  from  the  metalliferous  veins  which  tra- 
verse the  crust,  and  which  are  commonly  supposed  to 
have  been  filled  from  below. 

Evidence  of  Internal  Heat. — In  the  evidence 
obtainable  as  to  the  former  history  of  the  earth,  no  fact 
is  of  more  importance  than  the  existence  of  a  high  temper- 
ature beneath  the  crust,  which  has  now  been  placed  beyond 
all  doubt.  This  feature  of  the  planet's  organization  is  made 
clear  by  the  following  proofs: 

(1.)  Volcanoes. — In  many  regions  of  the  earth's  surface, 
openings  exist  from  which  steam  and  hot  vapors,  ashes  and 
streams  of  molten  rock,  are  from  time  to  time  emitted.  The 
abundance  and  wide  diffusion  of  these  openings,  inexplica- 
ble by  any  mere  local  causes,  must  be  regarded  as  indicative 
of  a  very  high  internal  temperature.  If  to  the  still  active 
vents  of  eruption,  we  add  those  which  have  formerly  been 
the  channels  of  communication  between  the  interior  and 
the  surface,  there  are  perhaps  few  large  regions  of  the 
globe  where  proofs  of  volcanic  action  cannot  be  found. 
Everywhere  we  meet  with  masses  of  molten  rock  which 
have  risen  from  below,  as  if  from  some  general  reservoir. 
The  phenomena  of  active  volcanoes  are  fully  discussed  in 
Book  III.  Part  I. 

(2.)  Hot  Springs. — Where  volcanic  eruptions  have  ceased, 
evidence  of  a  high  internal  temperature  is  still  often  to  be 
found  in  springs  of  hot  water  which  continue  for  centuries 
to  maintain  their  heat.  Thermal  springs,  however,  are  not 
confined  to  volcanic  districts.  They  sometimes  rise  even 
in  regions  many  hundreds  of  miles  distant  from  any  active 
volcanic  vent.  The  hot  springs  of  Bath  (temp.  120°  Fahr.) 
and  Buxton  (temp.  82°  Fahr.)  in  England  are  fully  900 


GEOGNOSY  93 

miles  from  the  Icelandic  volcanoes  on  the  one  side,  and 
1100  miles  from  those  of  Italy  and  Sicily  on  the  other. 

(3.)  Borings,  Wells  and  Mines. — The  influence  of  the 
seasonal  changes  of  temperature  extends  downward  from 
the  surface  to  a  depth  which  varies  with  latitude,  with 
the  thermal  conductivity  of  soils  and  rocks,  and  perhaps 
with  other  causes.  The  cold  of  winter  and  the  heat  of 
summer  may  be  regarded  as  following  each  other  in  suc- 
cessive waves  downward,  until  they  disappear  along  a  limit 
at  which  the  temperature  remains  constant.  This  zone  of 
invariable  temperature  is  commonly  believed  to  lie  at  a 
depth  of  somewhere  between  60  and  80  feet  in  temperate 
regions.  At  Yakutsk  in  Eastern  Siberia  (lat.  62°  N.), 
however,  as  shown  in  a  well-sinking,  the  soil  is  perma- 
nently frozen  to  a  depth  of  about  700  feet.34  In  Java,  on 
the  other  hand,  a  constant  temperature  is  said  to  be  met 
with  at  a  depth  of  only  2  or  3  feet." 

It  is  a  remarkable  fact,  now  verified  by  observation  all 
over  the  world,  that  below  the  limit  of  the  influence  of 
ordinary  seasonal  changes  the  temperature,  so  far  as  we  yet 
know,  is  nowhere  found  to  diminish  downward.  It  always 
rises;  and  its  rate  of  increment  never  falls  much  below  the 
average.  The  only  exceptional  cases  occur  under  circum- 
stances not  difficult  of  explanation.  On  the  one  hand,  the 
neighborhood  of  hot-springs,  of  large  masses  of  lava,  or  of 
other  manifestations  of  volcanic  activity,  may  raise  the 
subterranean  temperature  much  above  its  normal  condi- 
tion; and  this  augmentation  may  not  disappear  for  many 
thousand  years  after  the  volcanic  activity  has  wholly 
ceased,  since  the  cooling  down  of  a  subterranean  mass  of 

34  Helmereen,  Brit.  Assoc.  Rep.  1871,  p.  22.     See  vol.  for  1886,  p.  271. 
45  Junghuhn's  "Java,"  ii.  p.  771. 


&t  TEXT-BOOK   OF   GEOLOGY 

lava  must  necessarily  be  a  very  slow  process.  Lord  Kelvin 
has  even  proposed  to  estimate  the  age  of  subterranean 
masses  of  intrusive  lava  from  their  excess  of  temperature 
above  the  normal  amount  for  their  isogeotherms  (lines  of 
equal  earth-temperature),  some  probable  initial  temperature 
and  rate  of  cooling  being  assumed.  On  the  other  hand,  the 
spread  of  a  thick  mass  of  snow  and  ice  over  any  consider- 
able area  of  the  earth's  surface,  and  its  continuance  there 
for  several  thousand  years,  would  so  depress  the  isogeo- 
therms that,  for  many  centuries  afterward,  there  would 
be  a  fall  of  temperature  for  a  certain  distance  downward. 
At  the  present  day,  in  at  least  the  more  northerly  parts 
of  the  northern  hemisphere,  there  are  such  evidences  of 
a  former  more  rigorous  climate,  as  in  the  well-sinking 
at  Yakutsk  just  referred  to.88  Lord  Kelvin  (Sir  W.  Thom- 
son)87 has  calculated  that  any  considerable  area  of  the 
earth's  surface  covered  for  several  thousand  years  by 
snow  or  ice,  and  retaining,  after  the  disappearance  of 
that  frozen  covering,  an  average  surface  temperature 
of  13°  C.,  "would  during  900  years  show  a  decreasing 
temperature  for  some  depth  down  from  the  surface,  and 
3600  years  after  the  clearing  away  of  the  ice  would  still 
show  residual  effect  of  the  ancient  cold,  in  a  half  rate  of 
augmentation  of  temperature  downward  in  the  upper  strata, 
gradually  increasing  to  the  whole  normal  rate,  which  would 
be  sensibly  reached  at  a  depth  of  600  metres." 

Beneath  the  limit  to  which  the  influence  of  the  changes 
of  the  seasons  extends,  observations  all  over  the  globe,  and 

36  Professor  Prestwich  (Inaugural  Lecture,  1875,  p.  45)  has  suggested  that 
to  the  more  rapid  refrigeration  of  the  earth's  surface  during  this  cold  period,  and 
to  the  consequent  depression  of  the  subterranean  isothermal  lines,  the  alleged 
present  comparative  quietude  of  the  volcanic  forces  is  to  be  attributed,  the  inter- 
nal heat  not  having  yet  recovered  its  dominion  in  the  outer  crust. 

31  Brit.  Assoc.  Reports,  1876,  Sections,  p.  3. 


GEOGNOSY  95 

at  many  different  elevations,  give  a  rate  of  increase  of  tem- 
perature downward,  or  "temperature  gradient,"  which  has 
been  usually  taken  to  be  1°  Fahr.  for  every  50  or  60  feet 
of  descent,  this  computation  being  based  especially  on  ob- 
servations in  deep  mines  and  borings.  Professor  Prestwich 
concluded  from  a  large  series  of  observations  collated  by 
him,  that  the  average  increment  might  be  taken  at  1°  Fahr. 
for  every  45  feet."  Observations  taken  in  the  extraordi- 
narily deep  boring  at  Schladebach,  near  Diirrenberg, 
showed  that  in  a  depth  of  5736  feet  the  average  rise  of 
temperature  was  1°  Fahr.  for  every  65  feet.89  According 
to  data  collected  by  a  Committee  of  the  British  Associa- 
tion, the  average  gradient  appears  to  be  1°  Fahr.  for  every 
64  feet,  or  i  of  a  degree  per  foot. 

Isogeotherms  near  the  surface  follow  approximately  the 
contours  of  the  surface,  but  are  flatter  than  these,  and  "their 
flattening  increases  as  we  pass  to  lower  ones,  until  at  a  con- 
siderable depth  they  become  sensibly  horizontal  planes. 
The  temperature  gradient  is  consequently  steepest  beneath 
gorges  and  least  steep  beneath  ridges. ' '  *° 

Irregularities  in  the  Downward  Incre- 
ment of  Heat. — While  there  is  everywhere  a  progres- 
sive increase  of  temperature  downward,  its  rate  is  by  no 
means  uniform.  The  more  detailed  observations  which 


*»  Proc.  Roy.  Soc.  xli.  (1885),  p.  55. 

39  Brit.  Assoc.  1889.     Report  of  Underground  Temperature  Committee. 

40  J.  D.  Everett,  Brit.  Assoc.  1879,  Sections,  p.  345.     Compare  also  the  elab- 
orate observations  made  in  the  St.  Gotuard  Tunnel,  F.  Stapff,  "Rapports,  Con- 
seil  Fed.  St.  Golhard,"  vol.  viii.,  and  "Geologische  Durchschnitte  des  Gothard 
Tunnels" ;  "Etude  de  I'Influence  de  la  Chaleur  de  1'Interieur  de  la  Terre,"  etc., 
Eevue.  Univ.  Mines,  1879-80.     Min.   Proc.  N.   England  Inst.  Mining-Mechan. 
Engin.  xxxii.  (1883),  p.  19.     "Reports  of  Committee  on  Underground  Tempera- 
ture," Brit.  Assoc.  Rep.  from  1868  onward,  with  summary  of  results  in  the  vol- 
ume for  1882.     A  voluminous  and  valuable  collection  of  data  bearing  on  this 
subject  was  compiled  by  Prof.  Prestwich  and  is  published  in  Proc.  Roy.  Soc. 
Xli.  (1885),  p.  1. 


96 


TEXT-BOOK   OF   GEOLOGY 


have  been  made  in  recent  years  have  brought  to  light  the 
important  act  that  considerable  variations  in  the  rate  of 
increase  take  place,  even  in  the  same  bore.  The  tempera- 
tures obtained  at  different  depths  in  the  Eose  Bridge  colliery 
shaft,  Wigan,  for  instance,  read  as  in  the  following  columns: 


Depth  in 
Yards 

558  .  .. 

Temperature 
(Fahr.) 

78 

746  

Tem^rat^ 
89 

605 

80 

761 

90| 

630 

83 

775 

663 

85 

783  

92 

671 

86 

800 

93 

679 

87 

806   .. 

.934 

734  

....884 

815.... 

....94 

At  La  Chapelle,  in  an  important  well  made  for  the  water- 
supply  of  Paris,  observations  have  been  taken  of  the  tem- 
perature at  different  depths,  as  shown  in  the  subjoined 

table:41 


pthin 
letres 

100 

Temperature 
(tfahr.) 
59-5 

Depth  in 

Metres 

500     

Temporal! 
(rfahr.) 
72-6 

200 

..  61'8 

600  

75-0 

300 

65*5 

660   

76-0 

400  

....69-0 

In  drawing  attention  to  the  foregoing  temperature- 
observations  at  the  Hose  Bridge  colliery — the  deepest 
mine  in  Great  Britain — Prof.  Everett  points  out  that,  as- 
suming the  surface  temperature  to  be  49°  Fahr.,  in  the 
first  558  yards,  the  rate  of  rise  of  temperature  is  1°  for 
67-7  feet;  in  the  next  257  yards  it  is  1°  in  48-2  feet;  in 
the  portion  between  605  and  671  yards — a  distance  of  only 
198  feet— it  is  1°  in  33  feet;  in  the  lowest  portion  of  432 
feet  it  is  1°  in  54  feet.48  When  such  irregularities  occur 
in  the  same  vertical  shaft,  it  is  not  surprising  that  the 
average  should  vary  so  much  in  different  places. 

41  Brit.  Assoc.  Eep.  1873,  Sections,  p.  254. 

42  Brit.  Assoc.  Rep.  1870,  Sections,  p.  31. 


GEOGNOSY  9 

There  can  be  little  doubt  that  one  main  cause  of  these 
variations  is  to  be  sought  in  the  different  thermal  conduc- 
tivities of  the  rocks  of  the  earth's  crust.  The  first  accurate 
measurements  of  the  conducting  powers  of  rocks  were  made 
by  the  late  J.  D.  Forbes  at  Edinburgh  (1837-1846).  He 
selected  three  sites  for  his  thermometers,  one  in  "trap- 
rock"  (a  porphyrite  of  Lower  Carboniferous  age),  one  in 
loose  sand,  and  one  in  sandstone,  each  set  of  instruments 
being  sunk  to  depths  of  8,  6,  12  and  24  French  feet  from 
the  surface.  He  found  that  the  wave  of  summer  heat 
reached  the  bulb  of  the  deepest  instrument  (24  feet)  on 
4th  January  in  the  trap-rock,  on  25th  December  in  the 
sand,  and  on  8d  November  in  the  sandstone,  the  trap-rock 
being  the  worst  conductor  and  the  solid  sandstone  by  far 
the  best.48 

As  a  rule,  the  lighter  and  more  porous  rocks  offer  the 
greatest  resistance  to  the  passage  of  heat,  while  the  more 
dense  and  crystalline  offer  the  least  resistance.  The  resist- 
ance of  opaque  white  quartz  is  expressed  by  the  number 
114,  that  of  basalt  stands  at  273,  while  that  of  cannel  coal 
stands  very  much  higher  at  1638,  or  more  than  thirteen 
times  that  of  quartz.44 

It  is  evident  also,  from  the  texture  and  structure  of  most 
rocks,  that  the  conductivity  must  vary  in  different  direc- 
tions through  the  same  mass,  heat  being  more  easily  con- 
ducted along  than  across  the  "grain,"  the  bedding,  and  the 
other  numerous  divisional  surfaces.  Experiments  have  been 
made  to  determine  these  variations  in  a  number  of  rocks. 
Thus  the  conductivity  in  a  direction  transverse  to  the  divi- 

43  Trans.  Roy.  Soc.  Edin.  xvi.  p.  211. 

44  Herschel  and  Lebour  (British  Association  Committee  on  Thermal  Conduc- 
tivities of  Rocks),  Brit.  Assoc.  Rep.  1876,  p.  69.     The  final  Report  is  in  the  vol 
for  1881. 

GEOLOGY— Vol.  XXIX— 6 


98  TEXT-BOOK   OF   GEOLOGY 

sional  planes  being  taken  as  unity,  the  conductivity  parallel 
with  these  planes  was  found  in  a  variety  of  magnesian  schist 
to  be  4-028.  In  certain  slates  and  schistose  rocks  from  cen- 
tral France,  the  ratio  varied  from  1  :  2-56  to  1  :  8-952. 
Hence  in  such  fissile  rocks  as  slate  and  mica-schist,  heat 
may  travel  four  times  more  easily  along  the  planes  of  cleav- 
age or  foliation  than  across  them.4* 

In  reasoning  upon  the  discrepancies  in  the  rate  of  in- 
crease of  subterranean  temperatures,  we  must  also  bear  in 
mind  that  convection  by  percolating  streams  of  water  must 
materially  affect  the  transference  of  heat  from  below."  Cer- 
tain kinds  of  rock  are  more  liable  than  others  to  be  charged 
with  water,  and,  in  almost  every  boring  or  shaft,  one  or 
more  horizons  of  such  water-bearing  rocks  are  met  with. 
The  effect  of  interstitial  water  is  to  diminish  thermal  resist- 
ance. Dry  red  brick  has  its  resistance  lowered  from  680  to 
405  by  being  thoroughly  soaked  in  water,  its  conductivity 
being  thus  increased  68  per  cent.  A  piece  of  sandstone  has 
its  conductivity  heightened  to  the  extent  of  8  per  cent  by 
being  wetted.47  Q, 

Mallet  contended  that  the  variations  in  the  amount  of  in- 
crease in  subterranean  temperature  are  too  great  to  permit 
us  to  believe  them  to  be  due  merely  to  differences  in  the 
transmission  of  the  general  internal  heat,  and  that  they 
point  to  local  accessions  of  heat  arising  from  transforma- 
tion of  the  mechanical  work  of  compression,  which  is  due 

45  Report  of  Committee  on  Thermal  Conduclivities  of  Rocks,  Brit.  Assoc.  Rep. 
1875,  p.  61.  Jannettaz,  Bull.  Soc.  Geol.  France  (April-June,  1874),  ii.  p.  264. 
This  observer  has  carried  out  a  series  of  detailed  researches  on  the  propagation 
of  heat  through  rocks  which  will  be  found  in  Bull.  Soc.  Geol.  France,  tomes 
i.-ix.  (3d  series). 

48  In  the  great  bore  of  Sperenberg  (4172  feet,  entirely  in  rock-salt,  except 
the  first  283  feet)  there  is  evidence  that  the  water  near  the  top  is  warmed  4^° 
Pahr.  by  convection.  Brit.  Assoc.  1882,  p.  78. 

47  Herschel  and  Lebour,  Brit.  Assoc.  Rep.  1875,  p.  68. 


GEOGNOSY  99 

to  the  constant  cooling  and  contraction  of  the  globe.48  But 
it  may  be  replied  that  these  variations  are  not  greater  than, 
from  the  known  divergences  in  the  conductivities  of  rocks, 
they  might  fairly  be  expected  to  be. 

Probable  Condition  of  the  Earth's  Inte- 
rior.— Various  theories  have  been  propounded  on  this  sub- 
ject. There  are  only  three  which  merit  serious  considera- 
tion. (1.)  One  of  these  supposes  the  planet  to  consist  of  a 
solid  crust  and  a  molten  interior.  (2.)  The  second  holds 
that,  with  the  exception  of  local  vesicular  spaces,  the  globe 
is  solid  and  rigid  to  the  centre.  (3.)  The  third  contends 
that  while  the  mass  of  the  globe  is  solid,  there  lies  a  liquid 
substratum  beneath  the  crust. 

1.  The  arguments  in  favor  of  internal  liquidity  may  be 
summed  up  as  follows,  (a.)  The  ascertained  rise  of  tem- 
perature inward  from  the  surface  is  such  that,  at  a  very 
moderate  depth,  the  ordinary  melting-point  of  even  the  most 
refractory  substances  would  be  reached.  At  20  miles  the 
temperature,  if  it  increases  progressively,  as  it  does  in  the 
depths  accessible  to  observation,  must  be  about  1760°  Fahr. ; 
at  60  miles  it  must  be  4600°,  or  far  higher  than  the  fusing- 
point  even  of  so  stubborn  a  metal  as  platinum,  which  melts 
at  3080°  Fahr.4*  (6.)  All  over  the  world  volcanoes  exist 
from  which  steam  and  torrents  of  molten  lava  are  from  time 
to  time  erupted.  Abundant  as  are  the  active  volcanic  vents, 
they  form  but  a  small  proportion  of  the  whole  which  have 
been  in  operation  since  early  geological  time.  It  has  beea 
inferred,  therefore,  that  these  numerous  funnels  of  comma- 


48  "Volcanic  Energy,"  Phil.  Trans.  1876. 

49  But  Lord  Kelvin  (Sir  W.  Thomson)  has  shown  that  if  the  rate  of  increase 
of  temperature  is  taken  to  be  1°  for  every  51  feet  for  the  first  100,000  feet,  it 
will  begin  to  diminish  below  that  limit,  being  only  1°  in  2550  feet  at  800,000 
feet,  and  then  rapidly  lessening.     Trans.  Boy.  Soc.  Edin.  xxiii.  p.  163. 


100  TEXT-BOOK    OF   GEOLOGY 

nication  with  the  heated  interior  could  not  have  existed  and 
poured  forth  such  a  vast  amount  of  molten  rock,  unless  thej 
drew  their  supplies  from  an  immense  internal  molten  nu- 
cleus, (c.)  When  the  products  of  volcanic  action  from  dif- 
ferent and  widely-separated  regions  are  compared  and  ana- 
lyzed, they  are  found  to  exhibit  a  remarkable  uniformity  of 
character.  Lavas  from  Vesuvius,  from  Hecla,  from  the 
Andes,  from  Japan,  and  from  New  Zealand  present  such 
an  agreement  in  essential  particulars  as,  it  is  contended,  can 
only  be  accounted  for  on  the  supposition  that  they  have  all 
emanated  from  one  vast  common  source.60  (c?.)  The  abun- 
dant earthquake-shocks  which  affect  large  areas  of  the  globe 
are  maintained  to  be  inexplicable  unless  on  the  supposition 
of  the  existence  of  a  thin  and  somewhat  flexible  crust. 
These  arguments,  it  will  be  observed,  are  only  of  the  na- 
ture of  inferences  drawn  from  observations  of  the  present 
constitution  of  the  globe.  They  are  based  on  geological 
data,  and  have  been  frequently  urged  by  geologists  as  sup- 
porting the  only  view  of  the  nature  of  the  earth's  interior, 
supposed  by  them  to  be  compatible  with  geological  evi- 
dence. 

2.  The  arguments  in  favor  of  the  internal  solidity  of  the 
earth  are  based  on  physical  and  astronomical  considerations 
of  the  greatest  importance.  They  may  be  arranged  as  fol- 
lows: 

(a.)  Argument  from  precession  and  nutation. — The  prob- 
lem of  the  internal  condition  of  the  globe  was  attacked  as 
far  back  as  the  year  1839  by  Hopkins,  who  calculated  how 
far  the  planetary  motions  of  precession  and  nutation  would 
be  influenced  by  the  solidity  or  liquidity  of  the  earth's  into- 

«»  See  D.  Forbes,  Popular  Science  Review,  April,  1869. 


GEOGNOSY  101 

rior.  He  found  that  the  processional  and  nutational  move- 
ments could  not  possibly  be  as  they  are,  if  the  planet  con- 
sisted of  a  central  core  of  molten  rock  surrounded  with  a 
crust  of  twenty  or  thirty  miles  in  thickness;  that  the  least 
possible  thickness  of  crust  consistent  with  the  existing 
movements  was  from  800  to  1000  miles;  and  that  the  whole 
might  even  be  solid  to  the  centre,  with  the  exception  of 
comparatively  small  vesicular  spaces  filled  with  melted 
rock." 

M.  Delaunay"  threw  doubt  on  Hopkins'  views,  and  sug- 
gested that,  if  the  interior  were  a  mass  of  sufficient  viscos- 
ity, it  might  behave  as  if  it  were  a  solid,  and  thus  the 
phenomena  of  precession  and  nutation  might  not  be  af- 
fected. Lord  Kelvin  (Sir  W.  Thomson),  who  had  already 
arrived  at  the  conclusion  that  the  interior  of  the  globe  must 
be  solid,  and  acquiesced  generally  in  Hopkins'  conclusions, 
remarked  that  the  hypothesis  of  a  viscous  and  quasi-rigid 
interior  "breaks  down  when  tested  by  a  simple  calculation 
of  the  amount  of  tangential  force  required  to  give  to  any 
globular  portion  of  the  interior  mass  the  precessional  and 
nutational  motions  which,  with  other  physical  astronomers, 
M.  Delaunay  attributes  to  the  earth  as  a  whole.*'  He  held 
the  earth's  crust  down  to  depths  of  hundreds  of  kilometres 
to  be  capable  of  resisting  such  a  tangential  stress  (amounting 
to  nearly  ith  of  a  gramme  weight  per  square  centimetre)  as 
would  with  great  rapidity  draw  out  of  shape  any  plastic  sub- 
stance which  could  properly  be  termed  a  viscous  fluid,  and 

51  Phil.  Trans.  1839,  p.  381;  1840,  p.  193;  1842,  p.  43;  Brit.  Assoc.  1847. 

52  In  a  paper  on  the  hypothesis  of  the  interior  fluidity  of  the  globe,  Comptes 
rendus,  July  13,  1868.     Geol.  Mag.  v.  p.  507.     See  also  H.  Hennessy,  Comptes 
rendus,  March  6,  1871,  Geol.  Mag.  viii.  p.  216.     Nature,  xv.  p.  18.     0.  Fisher, 
•'Physics  of  the  Earth's  Crust,"  2d  Edition,  1889. 

53  Nature,  February  1,  1872. 


102  TEXT-BOOK    OF   GEOLOGY 

he  concluded  "that  the  rigidity  of  the  earth's  interior  sub- 
stance could  not  be  less  than  a  millionth  of  the  rigidity  of 
glass  without  very  sensibly  augmenting  the  lunar  nineteen- 
yearly  nutation.'4 

In  Hopkins'  hypothesis  he  assumed  the  crust  to  be  in- 
finitely rigid  and  unyielding,  which  is  not  true  of  any  ma- 
terial substance.  Lord  Kelvin  subsequently  returning  to 
the  problem,  in  the  light  of  his  own  researches  in  vortex- 
motion,  found  that,  while  the  argument  against  a  thin  crust 
and  vast  liquid  interior  is  still  invincible,  the  phenomena  of 
precession  and  nutation  do  not  decisively  settle  the  question 
of  internal  fluidity,  as  Hopkins,  and  others  following  him, 
had  believed,  though  the  solar  semi-annual  and  lunar  fort- 
nightly nutations  absolutely  disprove  the  existence  of  a  thin 
rigid  shell  full  of  liquid.  If  the  inner  surface  of  the  crust 
or  shell  were  rigorously  spherical,  the  interior  mass  of  sup- 
posed liquid  could  experience  no  processional  or  nutational 
influence,  except  in  so  far  as,  if  heterogeneous  in  composi- 
tion, it  might  suffer  from  external  attraction  due  to  non- 
sphericity  of  its  surfaces  of  equal  density.  But  "a  very 
slight  deviation  of  the  inner  surface  of  the  shell  from  per- 
fect sphericity  would  suffice,  in  virtue  of  the  quasi-rigidity 
due  to  vortex-motion,  to  hold  back  the  shell  from  taking 
sensibly  more  precession  than  it  would  give  to  the  liquid, 
and  to  cause  the  liquid  (homogeneous  or  heterogeneous)  and 
the  shell  to  have  sensibly  the  same  precessional  motion  as  if 
the  whole  constituted  one  rigid  body.""  The  problem  pre- 
sented by  the  precession  of  a  viscous  spheroid  has  more  re- 
cently been  discussed  by  Prof.  George  Darwin,  who  arrives 
at  results  nearly  the  same  as  those  announced  by  Lord  Kel- 

54  Loc.  ctt.  p.  258. 

55  Lord  Kelvin  (Sir  W.  Thomson),  Brit.  Assoc.  Rep.  1876,  Sections,  p.  5. 


GEOGNOSY  103 

vin  regarding  the  slight  difference  between  the  precession  of 
a  fluid  and  a  rigid  spheroid." 

The  assumption  of  a  comparatively  thin  crust  requires 
that  the  crust  shall  have  such  perfect  rigidity  as  is  possessed 
by  no  known  substance.  The  tide-producing  force  of  the 
moon  and  sun  exerts  such  a  strain  upon  the  substance  of 
the  globe,  that  it  seems  in  the  highest  degree  improbable 
that  the  planet  could  maintain  its  shape  as  it  does  unless  the 
supposed  crust  were  at  least  2000  or  2500  miles  in  thick- 
ness.*7 That  the  solid  mass  of  the  earth  must  yield  to  this 
strain  is  certain,  though  the  amount  of  deformation  is  so 
slight  as  to  have  hitherto  escaped  all  attempts  to  detect  it." 
Had  the  rigidity  been  even  that  of  glass  or  of  steel,  the  de- 
formation would  probably  have  been  by  this  time  detected, 
and  the  actual  phenomena  of  precession  and  nutation,  as 
well  as  of  the  tides,  would  then  have  been  very  sensibly 
diminished. M  The  conclusion  is  thus  reached  that  the  mass 
of  the  earth  "is  on  the  whole  more  rigid  certainly  than  a 
continuous  solid  globe  of  glass  of  the  same  diameter."6* 

(ft.)  Argument  from  the  tides. — The  phenomena  of  the 
oceanic  tides  show  that  the  earth  acts  as  a  rigid  body  either 
solid  to  the  centre,  or  possessing  so  thick  a  crust  (2500  miles 
or  more)  as  to  give  to  the  planet  practical  solidity.  Lord 
Kelvin  remarks  that  "were  the  crust  of  continuous  steel  and 
500  kilometres  thick,  it  would  yield  very  nearly  as  much  as 
if  it  were  India-rubber  to  the  deforming  influences  of  cen- 
trifugal force,  and  of  the  sun's  and  moon's  attractions."  It 
would  yield,  indeed,  so  freely  to  these  attractions  "that  it 


56  Phil.  Trans.  1879,  Part  2,  p.  464. 

61  Lord  Kelvin,  Proc.  Roy.  Soc.  April,  1862. 

58  See  Association  Francaise  pour  1'Avancement  des  Sciences,  v.  p.  281. 

59  Lord  Kelvin,  loc.  cit. 

60  Ibid.  Trans.  Roy.  Soc.  Edin.  xxiii.  p.  157. 


104  TEXT-BOOK   OF  QEOLOQY 

would  simply  carry  the  waters  of  the  ocean  up  and  down 
with  it,  and  there  would  be  no  sensible  tidal  rise  and  fall  of 
water  relatively  to  land."  Prof.  Gr.  II.  Darwin,  in  the  series 
of  papers  already  referred  to,  has  investigated  mathemati- 
cally the  bodily  tides  of  viscous  and  semi-elastic  spheroids, 
and  the  character  of  the  ocean  tides  on  a  yielding  nucleus.** 
His  results  tend  to  increase  the  force  of  Sir  "W  illiam  Thom- 
son's argument,  since  they  show  that  "no  very  considerable 
portion  of  the  interior  of  the  earth  can  even  distantly  ap- 
proach the  fluid  condition,"  the  effective  rigidity  of  the 
whole  globe  being  very  great. 

(c.)  Argument  from  relative  densities  of  melted  and  solid 
rock. — The  two  preceding  arguments  must  be  considered 
decisive  against  the  hypothesis  of  a  thin  shell  or  crust  cov- 
ering a  nucleus  of  molten  matter.  It  has  been  further 
urged,  as  an  objection  to  this  hypothesis,  that  cold  solid 
rock  is  more  dense  than  hot  melted  rock,  and  that  even  if 
a  thin  crust  were  formed  over  the  central  molten  globe  it 
would  immediately  break  up  and  the  fragments  would  sink 
toward  the  centre."  Recent  experiments  show  that  diabase 
(of  density  8*017)  contracts  nearly  4  per  cent  on  solidifica- 
tion, and  that  the  resulting  homogeneous  glass  has  a  density 
of  only  2-717.64  As  has  been  already  pointed  out,  the  spe- 
cific gravity  of  the  interior  is  at  least  twice  as  much  as  that 
of  the  visible  parts  of  the  crust.  If  this  difference  be  due, 
not  merely  to  the  effect  of  pressure,  but  to  the  presence  in  the 
interior  of  intensely  heated  metallic  substances,  we  cannot 

61  Lord  Kelvin,  Brit.  Assoc.  Rep.  1876,  Sections,  p.  7. 

62  Phil.  Trans.  1879,  Part  2.     See  also  Brit.  Assoc.  Rep.  1882,  Sects,  p.  473. 

63  This  objection  has  been  repeatedly  urged  by  Lord  Kelvin.     See  Trans. 
Roy.  Soc.  Edin.  xxiii.  p.  157;  and  Brit.  Assoc.  Rep.  1870,  Sections,  p.  7. 

64  C.  Barus,  Phil.  Mag.  1893,  p.  174.     It  is  nevertheless  true  that,  from  a 
cause  merely  mechanical,  pieces  of  the  original  cold   rock,  though  so  much 
denser,  will  iloat  for  a  time  on  the  melted  material.     Ib.  p.  189. 


GEOGNOSY  105 

suppose  that  solidified  portions  of  such  rocks  as  granite  and 
the  various  lavas  could  ever  have  sunk  into  the  centre  of  the 
earth,  so  as  to  build  up  there  the  honey-combed  cavernous 
mass  which  might  have  served  as  a  nucleus  in  the  ultimate 
solidification  of  the  whole  planet.  If  the  earliest  formed 
portions  of  the  comparatively  light  crust  were  denser  than 
the  underlying  liquid,  they  would  no  doubt  descend  until 
they  reached  a  stratum  with  specific  gravity  agreeing  with 
their  own,  or  until  they  were  again  melted." 

3.  Hypothesis  of  a  liquid  substratum  between  a  solid  nucleus 
and  the  crust. — Since  the  early  and  natural  belief  in  the 
liquidity  of  the  earth's  interior  has  been  so  weightily  op- 
posed by  physical  arguments,  geologists  have  endeavored 
to  modify  it  in  such  a  way  as,  if  possible,  to  satisfy  the  re- 
quirements of  physics,  while  at  the  same  time  providing  an 
adequate  explanation  of  the  corrugation  of  the  earth's  crust, 
the  phenomena  of  volcanoes,  etc."  The  hypothesis  has 
been  proposed  of  "a  rigid  nucleus  nearly  approaching  the 
size  of  the  whole  globe,  covered  by  a  fluid  substratum  of  no 
great  thickness,  compared  with  the  radius,  upon  which  a 
crust  of  lesser  density  floats  in  a  state  of  equilibrium."  The 
nucleus  is  assumed  to  owe  its  solidity  to  "the  enormous 
pressure  of  the  superincumbent  matter,  while  the  crust  owes 
its  solidity  to  having  become  cool.  The  fluid  substratum  is 


M  See  D.  Forbes,  Geol.  Mag.  vol.  iv.  p.  435.  The  evidence  for  the  internal 
solidity  of  the  earth  is  criticised  by  Dr.  M.  E.  Wadsworth  in  the  American 
Naturalist,  1884. 

64  See  Dana  in  Silliman's  Journal,  iii.  (1847),  p.  147.  Amer.  Journ.  Science 
(1873).  The  hypothesis  of  a  fluid  substratum  has  been  advocated  by  Shaler. 
Proc.  Bost.  Nat.  Hist.  Soc.  xi.  (1868),  p.  8.  Geol.  Mag.  v.  p.  511.  J.  Le  Conte, 
Amer.  Journ.  Sci.  1872,  1873.  0.  Fisher,  Geol.  Mag.  v.  (new  series),  pp.  291 
and  551.  "Physics  of  the  Earth's  Crust,"  1883.  [This  author  in  his  second 
edition  modifies  this  view.]  Hill,  Geol.  Mag.  v.  (new  series),  pp.  262,  479.  The 
idea  of  a  viscous  layer  between  the  solidifying  central  mass  and  the  crust  was 
present  in  Hopkins'  mind.  Brit.  Assoc.  1848,  Reports,  p.  48. 


106  TEXT-BOOK   OF   GEOLOGY 

not  under  sufficient  pressure  to  be  rendered  solid,  and  is 
sufficiently  hot  to  be  fluid,  being  probably  more  viscous  in 
its  lower  portion  through  pressure  and  likewise  passing  into 
a  viscous  state  in  its  upper  parts  through  cooling,  until  it 
joins  the  crust."'7  The  contraction  and  consolidation  of  this 
substratum  are  assumed  as  the  explanation  of  the  plication 
which  the  crust  has  certainly  undergone. 

It  must  be  admitted  that  the  widespread  proofs  of  great 
crumpling  of  the  rocks  of  the  crust  present  a  difficulty,  for 
they  indicate  a  capability  of  yielding  to  strain  such  as  has 
been  supposed  impossible  in  a  globe  possessing  on  the 
whole  the  rigidity  of  steel  or  glass.  But  this  difficulty 
may  be  more  formidable  in  appearance  than  in  reality. 
The  earth  must  certainly  possess  sjich  a  degree  of  rigidity 
as  to  resist  tidal  deformation.  Prof.  Darwin  has  calculated 
the  limiting  rigidity  in  the  materials  of  the  earth  which  is 
necessary  to  prevent  the  weight  of  mountains  and  continents 
from  reducing  them  to  the  fluid  condition  or  else  cracking, 
and  has  found  that  these  materials  must  be  as  strong  as 
granite  1000  miles  below  the  surface,  or  else  much  stronger 
than  granite  near  the  surface."  But  high  rigidity,  that  is, 
elasticity  of  form,  is  not  contradictory  of  plasticity.  Even 
bodies  like  steel  may,  under  suitable  stress,  be  made  to  flow 
like  butter  (see  postea,  Book  III.  Part  I.  Sect.  iv.  §  3). 
While,  therefore,  the  earth  may  possess  as  a  whole  the 
rigidity  of  steel,  there  seems  no  reason  why,  under  suffi- 
cient strain,  the  outer  portions  may  not  be  plicated  or 
even  reduced  to  the  fluid  condition.  It  is  important  "to 
distinguish  viscosity,  in  which  flow  is  caused  by  infini- 
tesimal forces,  from  plasticity,  in  which  permanent  distor- 

61  Fisher,  "Physics  of  Earth's  Crust,"  1st  edit.  p.  269. 
68  Proc.  Roy,  Soc.  1881,  p.  432. 


GEOGNOSY  107 

tion  or  flow  only  sets  in  when  the  stresses  exceed  a  certain 
limit."69 

In  speculating  on  the  plication  of  the  earth's  crust,  we 
ought  not  to  forget  that,  from  the  earliest  times,  the  exist- 
ing continental  regions  seem  to  have  specially  suffered  from 
the  efforts  of  the  planet  to  adjust  its  external  form  to  its 
diminishing  diameter  and  lessening  rapidity  of  rotation. 
They  have  served  as  lines  of  relief  from  the  strain  of  com- 
pression during  many  successive  epochs.  It  is  along  their 
axial  lines — their  long  dominant  mountain-ranges,  that  we 
should  naturally  look  for  evidence  of  corrugation.  Away 
from  these  lines  of  weakness  the  ground  has  been  upraised 
for  thousands  of  square  miles  without  plication  of  the  rocks, 
as  in  the  instructive  region  of  the  Western  Territories  of 
North  America.  Nor  is  there  any  proof  that  corrugation 
takes  place  beneath  the  great  oceanic  areas  of  subsidence. 

It  appears  highly  probable  that  the  substance  of  the 
earth's  interior  is  at  the  melting-point  proper  for  the  press- 
ure at  each  depth.  Any  relief  from  pressure,  therefore, 
may  allow  of  the  liquefaction  of  the  matter  so  relieved. 
Such  relief  is  doubtless  afforded  by  the  corrugation  of 
mountain-chains  and  other  terrestrial  ridges.  And  it  is 
in  these  lines  of  uprise  that  volcanoes  and  other  manifes- 
tations of  subterranean  heat  actually  show  themselves. 

§  4.  Age  of  the  Earth  and  Measures  of  Geological  Tipie. 
— The  age  of  our  planet  is  a  problem  which  may  be  at- 
tacked either  from  the  geological  or  physical  side. 

1.  The  geological  arguments  rest  chiefly  upon 
the  observed  rates  at  which  geological  changes  are  being 
effected  at  the  present  time,  and  is  open  to  the  obvious 
preliminary  objection  that  it  assumes  the  existing  rate  of 

69  Prof.  Darwin  in  a  letter  to  the  author,  9th  January,  1884. 


108  TEXT-BOOK    OF   GEOLOGY 

change  as  the  measure  of  past  revolutions — an  assumption, 
however,  which  may  be  erroneous,  for  the  present  may  be 
a  period  when  all  geological  events  march  forward  more 
slowly  than  they  used  to  do.  The  argument  proceeds  on 
data  partly  of  a  physical  and  partly  of  an  organic  kind, 
(a.)  The  physical  evidence  is  derived  from  such  facts  as 
the  observed  rates  at  which  the  surface  of  a  country 
is  lowered  by  rain  and  streams,  and  new  sedimentary  de- 
posits are  formed.  These  facts  will  be  more  particularly 
dwelt  upon  in  later  sections  of  this  work.  If  we  assume 
that  the  land  has  been  worn  away,  and  that  stratified 
deposits  have  been  laid  down,  nearly  at  the  same  rate  as 
at  present,  then  we  must  admit  that  the  stratified  portion 
of  the  crust  of  the  earth  must  represent  a  very  vast  period 
of  time.70  (6.)  On  the  other  hand,  human  experience,  so  far 
as  it  goes,  warrants  the  belief  that  changes  in  the  organic 
world  proceed  with  extreme  slowness.  Yet  in  the  stratified 
rocks  of  the  terrestrial  crust  we  have  abundant  proof  that 
the  whole  fauna  and  flora  of  the  earth's  surface  have  passed 
through  numerous  cycles  of  revolution — species,  genera, 
families,  orders,  appearing  and  disappearing  many  times 
in  succession.  On  any  supposition,  it  must  be  admitted 
that  these  vicissitudes  in  the  organic  world  can  only  have 

10  Dr.  Croll  put  this  period  at  not  less,  but  possibly  much  more,  than  60  mil- 
lion years.  Dr.  Haughton  gives  a  much  more  extended  period.  Estimating  the 
present  rate  of  deposit  of  strata  at  1  foot  in  8616  years,  assuming  the  former 
rate  to  have  been  ten  times  more  rapid,  or  1  foot  in  861 '6  years,  and  taking  the 
thickness  of  the  stratified  rocks  of  the  earth's  crust  at  177,200  feet,  he  obtains 
a  minimum  of  200,000,000  years  for  the  whole  duration  of  geological  time:  "Six 
Lectures  on  Physical  Geography,"  1880,  p.  94.  Dr.  Haughton  has  also  pro- 
posed another  geological  measure  of  past  time,  based  upon  the  assumed  effects 
of  continental  upheaval  (Proc.  Roy.  Soc.  xxvi.  (1877),  p.  534).  But  Prof  Dar- 
win has  shown  it  to  be  inadmissible.  (Op.  cit.  xxvii.  (1878),  p.  179.)  For  vari- 
ous opinions  regarding  geological  measures  of  time  see  J.  Phillips,  Brit.  Assoe. 
1864:  Croll,  Phil.  Mag.  1868:  T.  McK.  Hughes,  Proc.  Roy.  Inst.  Great  Britain, 
March  24,  1876:  Dupont,  Bull.  Acad.  Roy.  Belgique,  viii.  (1834):  T.  Mellard 
Reade,  Quart.  Journ.  Geol.  Soc.  1888,  p.  291. 


GEOGNOSY  109 

been  effected  with  the  lapse  of  vast  periods  of  time,  though 
no  reliable  standard  seems  to  be  available  whereby  these 
periods  are  to  be  measured.  The  argument  from  geological 
evidence  indicates  an  interval  of  probably  not  much  less 
than  100  million  years  since  the  earliest  forms  of  life  ap- 
peared upon  the  earth,  and  the  oldest  stratified  rocks  began 
to  be  laid  down. 

2.  The  physical  argument  as  to  the  age  of  our 
planet  is  based  by  Lord  Kelvin  upon  three  kinds  of  evi- 
dence :  (1)  the  internal  heat  and  rate  of  cooling  of  the  earth ; 
(2)  the  tidal  retardation  of  the  earth  rotation ;  and  (3)  the 
origin  and  age  of  the  sun's  heat. 

(1.)  Applying  Fourier's  theory  of  thermal  conductivity, 
he  pointed  out  as  far  back  as  the  year  1862,  that  in  the 
known  rate  of  increase  of  temperature  downward  beneath 
the  surface,  and  the  rate  of  loss  of  heat  from  the  earth,  we 
have  a  limit  to  the  antiquity  of  the  planet.  He  showed,  from 
the  data  available  at  the  time,  that  the  superficial  consolida- 
tion of  the  globe  could  not  have  occurred  less  than  20  mil- 
lion years  ago,  or  the  underground  heat  would  have  been 
greater  than  it  is;  nor  more  than  400  million  years  ago, 
otherwise  the  underground  temperature  would  have  shown 
no  sensible  increase  downward.  He  admitted  that  very 
wide  limits  were  necessary.  In  subsequently  discussing 
the  subject,  he  inclined  rather  toward  the  lower  than  the, 
higher  antiquity,  but  concluded  that  the  limit,  from  a  con- 
sideration of  all  the  evidence,  must  be  placed  within  some 
such  period  of  past  time  as  100  millions  of  years.  He  would 
now  restrict  the  time  to  about  20  millions.71 

11  Trans.  Roy.  Soc.  Edin.  xxiii.  p.  157.  Trans.  Geol.  Soc.  Glasgow,  iii.  p.  25. 
"Popular  Lectures  and  Addresses,"  2d  edit.  (1891),  p.  397.  Prof.  Tail  reduces 
the  period  to  10  or  15  millions.  "Recent  Advances  in  Physical  Science," 
p.  167. 


110  TEXT-BOOK   OF   GEOLOGY 

(2.)  The  reasoning  from  tidal  retardation  proceeds  on  the 
admitted  fact  that,  owing  to  the  friction  of  the  tide-wave, 
the  rotation  of  the  earth  is  retarded,  and  is  therefore  slower 
now  than  it  must  have  been  at  one  time.  Lord  Kelvin 
contends  that  had  the  globe  become  solid  some  10,000 
million  years  ago,  or  indeed  any  high  antiquity  beyond 
100  million  years,  the  centrifugal  force  due  to  the  more 
rapid  rotation  must  have  given  the  planet  a  very  much 
greater  polar  flattening  than  it  actually  possesses.  He  ad- 
mits, however,  that  though  100  million  years  ago  that  force 
must  have  been  about  3  per  cent  greater  than  now,  yet 
"nothing  we  know  regarding  the  figure  of  the  earth  and 
the  disposition  of  land  and  water  would  justify  us  in  say- 
ing that  a  body  consolidated  when  there  was  more  centrif- 
ugal force  by  3  per  cent  than  now,  might  not  now  be  in  all 
respects  like  the  earth,  so  far  as  we  know  it  at  present."  " 

(3.)  The  third  kind  of  evidence  leads  to  results  similar 
to  those  derived  from  the  two  previous  lines  of  reasoning. 
It  is  based  upon  calculations  as  to  the  amount  of  heat  that 
would  be  available  by  the  falling  together  of  masses  from 
space,  which  gave  rise  by  their  impact  to  our  sun,  and  the 
rate  at  which  this  heat  has  been  radiated.  Assuming  that 
the  sun  has  been  cooling  at  a  uniform  rate,  Prof.  Tait  con- 
cludes that  it  cannot  have  supplied  the  earth,  even  at  the 
present  rate,  for  more  than  about  15  or  20  million  years.7* 
Lord  Kelvin  also  believes  that  the  sun's  light  will  not  last 
more  than  6  or  6  millions  of  years  longer.7* 


TO  Trans.  Geol.  Soc.  Glasgow,  iii.  p.  16.  Prof.  Tait,  in  repeating  this  argu- 
ment, concludes  that,  taken  in  connection  with  the  previous  one,  "it  probably 
reduces  the  possible  period  which  can  be  allowed  to  geologists  to  something  less 
than  10  millions  of  years."  "Recent  Advances,"  p.  174.  Compare  Newcombe, 
"Popular  Astronomy,"  p.  505. 

™  Op.  cit.  p.  174.  M  "Popular  Lectures,"  etc.  p.  397. 


GEOGNOSY  HI 

There  can  be  no  doubt  that  the  demands  of  the  earlier 
geologists  for  an  unlimited  duration  of  past  time,  for  the 
accomplishment  of  geological  history,  were  extravagant  and 
unnecessary.  But  it  may  be  questioned  how  far  the  recent 
limitation  of  time  proposed  from  physical  considerations 
are  really  founded  on  well-established  facts.  The  argument 
from  the  geological  record  in  favor  of  a  much  longer  period 
than  physicists  are  disposed  to  concede  is  so  strong  that 
one  is  inclined  to  believe  that  these  writers  have  overstated 
their  case.  The  evidence  from  the  nature  of  the  sedimen- 
tary rocks,  and  from  the  succession  of  organic  remains  in 
these  rocks,  appears  to  me  to  demand  an  amount  of  time 
not  far  short  of  the  hundred  millions  of  years  originally 
granted  by  Lord  Kelvin." 

PART  II. — AN  ACCOUNT  OF  THE  COMPOSITION  OP  THE 
EARTH'S  CRUST — MINERALS  AND  BOCKS 

The  earth's  crust  is  composed  of  mineral  matter  in  va- 
rious aggregates  included  under  the  general  term  Bock. 
A  rock  may  be  denned  as  a  mass  of  matter  composed  of 
one  or  more  simple  minerals,  having  usually  a  variable 
chemical  composition,  with  no  necessarily  symmetrical  ex- 
ternal form,  and  ranging  in  cohesion  from  mere  loose  debris 
up  to  the  most  compact  stone.  Granite,  lava,  sandstone, 
limestone,  gravel,  sand,  mud,  soil,  marl  and  peat,  are  all 
recognized  in  a  geological  sense  as  rocks.  The  study  of 
rocks  is  known  as  Lithology,  Petrography  or  Petrology. 

It  will  be  most  convenient  to  treat — 1st,  of  the  general 
chemical  constitution  of  the  crust;  2d,  of  the  minerals  of 

15  I  have  touched  on  this  question  in  my  Presidential  Address  to  the  British 
Association  1892.  But  aee  a  paper  by  Mr.  Clarence  King,  Amer.  Journ.  Set 
xiy.  (1893). 


112  TEXT-BOOK   OF   GEOLOGY 

which  rocks  mainly  consist;  3d,  of  the  methods  employed 
for  the  determination  of  rocks;  4th,  of  the  external  char- 
acters of  rocks;  5th,  of  the  internal  texture  and  structure 
of  rocks;  6th,  of  the  classification  of  rocks;  and  7th,  of 
the  more  important  rocks  occurring  as  constituents  of  the 
earth's  crust. 

§  i.  General  Chemical  Constitution  of  the  Crust 
Direct  acquaintance  with  the  chemical  constitution  of 
the  globe  must  obviously  be  limited  to  that  of  the  crust, 
though  by  inference  we  may  eventually  reach  highly  prob- 
able conclusions  regarding  the  constitution  of  the  interior. 
Chemical  research  has  discovered  that  some  sixty-four1 
simple  or  as  yet  undecomposable  bodies,  called  elements, 
in  various  proportions  and  compounds,  constitute  the  ac- 
cessible part  of  the  crust.  Of  these,  however,  the  great 
majority  are  comparatively  of  rare  occurrence.  The  crust, 
go  far  as  we  can  examine  it,  is  mainly  built  up  of  about 
sixteen  elements,  which  may  be  arranged  in  the  two  follow- 
ing groups,  the  most  abundant  bodies  being  placed  first  in 
each  list: 


MetaMoids  At.  Wt. 

Oxygen 15-96 

Silicon 28-00 

Carbon 11-97 

31-98 

irogen 1-00 

....35-37 


Phosphorus 30-96 

Fluorine....  ....19-10 


Metals  At.  Wt. 

Aluminium. 27-30 

Calcium 39.90 

Magnesium. 23-94 

Potassium , 39-04 

Sodium 22-99 

Iron 55-90 

Manganese 54-80 

Barium....  ....136-80 


The  sixteen  elements  here  mentioned  form  about  ninety- 
nine  parts  of  the  earth's  crust;  the  other  elements  constitute 
only  about  a  hundredth  part,  though  they  include  gold,  sil- 

1  This  number  has  within  the  last  few  years  been  increased  by  the  alleged 
discovery  of  no  fewer  than  fourteen  new  metals  Some  of  these  bodies,  how- 
ever, have  not  yet  been  satisfactorily  proved  to  be  new.  T.  S.  Humpidge,  Nature, 
xxii.  p.  232. 


GEOGNOSY  113 

ver,  copper,  tin,  lead,  and  the  other  useful  metals,  iron  ex- 
cepted.  By  far  the  most  abundant  and  important  element 
is  Oxygen.  It  forms  about  23  per  cent  by  weight  of  air, 
88-87  per  cent  of  water,  and  about  a  half  of  all  the  rocks 
which  compose  the  visible  portion  or  crust  of  the  globe. 
Another  metalloid,  Silicon,  always  united  with  oxygen, 
ranks  next  in  abundance  as  a  constituent  of  the  crust.  Of 
the  remaining  metalloids,  Carbon  and  Sulphur  sometimes 
occur  in  the  free  state,  but  more  usually  in  combination. 
Chlorine  (save  perhaps  at  volcanic  vents)  does  not  occur 
in  a  free  state,  but  is  abundant  in  combination  with  the 
alkalies,  especially  with  sodium.  Fluorine  is  always  found 
in  combination,  and  has  only  recently  been  isolated  by  arti- 
ficial chemical  processes.  It  is  the  only  element  which  has 
not  been  combined  with  oxygen.  It  chiefly  occurs  in  union 
with  Calcium  as  the  mineral  fluor-spar,  and  constitutes  more 
than  half  of  the  mineral  cryolite;  but  traces  of  its  presence 
have  been  detected  in  other  minerals,  in  sea-water,  and  in 
the  bones,  teeth,  blood  and  milk  of  mammalia.  Hydrogen 
occurs  chiefly  in  combination  with  oxygen  as  the  oxide, 
water,  of  which  it  forms  11-18  per  cent  by  weight;  also  in 
combination  with  carbon  as  the  hydrocarbons  (mineral  oils 
and  gases),  produced  by  the  slow  decomposition  of  organic 
matter.  Phosphorus  occurs  with  oxygen  principally  in  cal- 
cic phosphate.  Of  the  metals,  a  few  are  found  in  the  native 
state  (gold,  silver,  copper,  etc),  but  those  of  importance  in 
the  framework  of  the  earth's  crust  have  entered  into  com- 
bination with  metalloids  or  with  each  other.  Putting  the 
more  important  metals  and  metalloids  together,  we  may 
compute  that  oxygen,  silicon,  aluminium,  magnesium,  cal- 
cium, potassium,  sodium,  iron  and  carbon,  form  together 
more  than  97  per  cent  of  the  whole  known  crust. 


114  TEXT-BOOK   OF   GEOLOGY 

So  far  as  accessible  to  observation,  the  outer  portion  of 
our  planet  consists  mainly  of  metalloids.  Its  metallic  con- 
stituents have  already  in  great  part  entered  into  combination 
with  oxygen,  so  that  the  atmosphere  contains  the  residue  of 
that  gas  which  has  not  yet  united  itself  to  terrestrial  com- 
pounds. In  a  broad  view  of  the  arrangement  of  the  chem- 
ical elements  in  the  external  crust,  the  suggestive  specula- 
tion of  Durocher  deserves  attention.*  He  regarded  all  rocks 
as  referable  to  two  layers  or  magmas  coexisting  in  the 
earth's  crust,  the  one  beneath  the  other,  according  to  their 
specific  gravities.  The  upper  or  outer  shell,  which  he 
termed  the  acid  or  siliceous  magma,  contains  an  excess  of 
silica,  and  has  a  mean  density  of  2-65.  The  lower  or  inner 
shell,  which  he  called  the  basic  magma,  has  from  six  to 
eight  times  more  of  the  earthy  bases  and  iron-oxides,  with 
a  mean  density  of  2 -96.  To  the  former  he  assigned  the  early 
plutonic  rocks,  granite,  felsite,  etc.,  with  the  more  recent 
trachytes;  to  the  latter  he  relegated  all  the  heavy  lavas, 
basalts,  diorites,  etc.  The  ratio  of  silica  is  7  in  the  acid 
magma  to  6  in  the  basic.  Though  the  proportion  of  silicic 
acid  or  of  the  earthy  and  metallic  bases  cannot  be  regarded 
as  any  certain  evidence  of  the  geological  date  of  rocks,  nor 
of  their  probable  depth  of  origin,  it  is  nevertheless  a  fact 
that  (with  many  important  exceptions)  the  eruptive  rocks  of 
the  older  geological  periods  are  very  generally  super-sili- 
cated  and  of  lower  specific  gravity,  while  those  of  later  time 
are  very  frequently  poor  in  silica,  but  rich  in  the  earthy 
bases  and  in  iron  and  manganese,  with  a  consequent  higher 
specific  gravity.  The  latter,  according  to  Durocher,  have 
been  forced  up  from  a  lower  zone  through  the  lighter  sili- 

9  Ann.  dee  Mines,  1857.  Translated  by  Haughton,  "Manual  of  Geology," 
1866,  p.  16. 


GEOGNOSY  115 

ceous  crust.  The  sequence  of  volcanic  rocks,  as  first  an- 
nounced by  Richthofen,  has  an  interesting  connection  with 
this  speculation.3 

The  main  mass  of  the  earth's  crust  is  composed  of  a  few 
predominant  compounds.  Of  these  in  every  respect  the 
most  abundant  and  important  is  Silicon-dioxide  or  Silica 
(Kieselerde)  SiOs.  As  the  fundamental  ingredient  of  the 
mineral  kingdom,  it  forms  more  than  one-half  of  the  known 
crust,  which  it  seems  to  bind  firmly  together,  entering  as  a 
main  ingredient  into  the  composition  of  most  crystalline  and 
fragmental  rocks  as  well  as  into  the  veins  that  traverse  them. 
It  occurs  in  the  free  state  as  the  abundant  rock-forming 
mineral  quartz,  which  strongly  resists  ordinary  decay,  and 
is  therefore  a  marked  constituent  of  many  of  the  more  en- 
during kinds  of  rock.  As  one  of  the  acid-forming  oxides 
(H4SiO4,  Silicic  acid,  Kieselsaure)  it  forms  combinations 
with  alkaline,  earthy,  and  metallic  bases,  which  appear  as 
the  prolific  and  universally  diffused  family  of  the  silicates. 
Moreover,  it  is  present  in  solution  in  terrestrial  and  oceanic 
waters,  from  which  it  is  deposited  in  pores  and  fissures  of 
rocks.  It  is  likewise  secreted  from  these  waters  by  abun- 
dantly diffused  species  of  plants  and  animals  (diatoms,  radio- 
larians,  etc.).  It  has  been  largely  effective  in  replacing  the 
organic  textures  of  former  organisms,  and  thus  preserving 
them  as  fossils.  » 

Alumina  or  aluminium-oxide  (Thonerde),  A18O3,  occurs 
sparingly  as  corundum,  which,  however,  according  to  F.  A. 
Genth,  was  the  original  condition  of  many  now  abundant 
complex  aluminous  minerals  and  rocks.  The  most  common 
condition  of  aluminium  is  in  union  with  silica.  In  this 

3  Posted,  Book  in.  Part  I.  Section  i.  §  5, 


116  TEXT-BOOK   OF   GEOLOGY 

form  it  constitutes  the  basis  of  the  vast  family  of  the  alumi- 
nous silicates,  of  which  so  large  a  portion  of  the  crystalline 
and  fragmental  rocks  consists.  Exposed  to  the  atmosphere, 
these  silicates  lose  some  of  their  more  soluble  ingredients, 
and  the  remainder  forms  an  earth  or  clay  consisting  chiefly 
of  silicate  of  aluminium. 

Carbon  is  the  fundamental  element  of  organic  life.  In 
combination  with  hydrogen,  as  well  as  with  oxygen,  nitrogen 
and  sulphur,  it  forms  the  various  kinds  of  coal,  and  thus 
takes  rank  as  an  important  rock-forming  element.  As  car- 
bon-dioxide, CO5,  it  is  present  in  the  air,  in  rain,  in  the  sea 
and  in  ordinary  terrestrial  waters.  This  oxide  is  soluble  in 
water,4  giving  rise  then  to  a  dibasic  acid  termed  Carbonic 
Acid  (Kohlensaure),  CO(OH)2  or  H2CO  ,  which  forms  carbo- 
nates, its  combination  with  calcium  having  been  instrumen- 
tal in  the  formation  of  vast  masses  of  solid  rock.  Carbon- 
dioxide  constitutes  a  fifth  part  of  the  weight  of  ordinary 
limestone. 

Sulphur  (Soufre,  Schwefel)  occurs  uncombined  in  occa- 
sional deposits  like  those  of  Sicily  and  Naples,  to  be  after- 
ward described,  also  in  union  with  iron  and  other  metals  as 
sulphides ;  but  its  principal  condition  as  a  rock-builder  is  in 
combination  with  oxygen  as  sulphuric  acid  (Schwefelsaure), 
HsSO4,  which  forms  sulphates  of  lime,  magnesia,  etc. 

Calcium  enters  into  the  composition  of  many  crystalline 
rocks  in  combination  with  silica  and  with  other  silicates. 
But  its  most  abundant  form  is  in  union  with  carbon -dioxide, 
when  it  appears  as  the  mineral,  calcite  (CaCO3),  or  the  rock, 
limestone.  Calcium-carbonate,  being  soluble  in  water  con- 
taining carbonic  acid,  is  one  of  the  most  universally  diffused 

4  One  volume  of  water  at  0°  C.  dissolves  1-7967  volumes  of  carbon-dioxide; 
at  15°  C.  the  amount  is  reduced.to  1-0020  volumes. 


GEOGNOSY  117 

mineral  ingredients  of  natural  waters.  It  supplies  the  varied 
tribes  of  mollusks,  corals,  and  many  other  invertebrates  with 
mineral  substance  for  the  secretion  of  their  tests  and  skele- 
tons. Such  too  has  been  its  office  from  remote  geological 
periods,  as  is  shown  by  the  vast  masses  of  organically- 
formed  limestone,  which  enter  so  conspicuously  into  the 
structure  of  the  continents.  In  combination  with  sulphuric 
acid,  calcium  forms  important  beds  of  gypsum  and  anhy- 
drite. 

Magnesium,  Potassium,  and  Sodium  play  a  less  conspicu- 
ous but  still  essential  part  in  the  composition  of  the  earth's 
crust.  Magnesium,  in  combination  with  silica,  forms  a  class 
of  silicates  of  prime  importance  in  the  composition  of  vol- 
canic and  metamorphic  rocks.  As  a  carbonate,  it  unites 
with  calcium-carbonate  to  form  the  widely  diffused  rock, 
dolomite.  In  union  with  chlorine,  it  takes  a  prominent 
place  among  the  salts  of  sea-water.  Potassium  or  Sodium, 
combined  with  silica,  is  present  in  small  quantity  in  most 
silicates.  In  union  with  chlorine,  as  common  salt,  sodium 
is  the  most  important  mineral  ingredient  of  sea-water,  and 
can  be  detected  in  minute  quantities  in  air,  rain,  and  in  ter- 
restrial waters.  In  the  old  chemical  formulae  hitherto  em- 
ployed in  mineralogy  the  metals  of  the  alkalies  and  alkaline 
earths  are  represented  as  oxides.  Thus  lime  (calcium-mon- 
oxide), soda  (sodium-monoxide),  potash  (potassium-monox- 
ide), magnesia  (magnesium-oxide),  are  denoted  as  in  union 
with  carbonic  acid,  sulphuric  acid,  silica,  etc.,  forming  car- 
bonates, sulphates,  silicates  of  lime,  soda,  etc. 

Iron  and  Manganese  are  the  two  most  common  heavy 
metals,  occurring  both  in  the  form  of  ores,  and  as  constitu- 
ents of  rocks.  Iron  is  the  great  pigment  of  nature.  Its 
peroxide  or  sesquioxide,  now  known  as  ferric  oxide,  forms 


118  TEXT-BOOK  OF    GEOLOGY 

large  mineral  masses,  and  together  with  the  protoxide  or 
ferrous  oxide,  occurs  in  smaller  or  larger  proportions  in  the 
great  majority  of  crystalline  rocks.  Iron  (as  sulphate  or  in 
combination  with  organic  acids)  is  removed  in  solution  in 
the  water  of  springs,  and  precipitated  as  a  hydrous  perox- 
ide. Manganese  is  commonly  associated  with  iron  in  mi- 
nute proportions  in  igneous  rocks,  and  being  similarly 
removed  in  solution  in  water,  is  thrown  down  as  bog 
manganese  or  wad. 

Silicic  Acid,  Carbonic  Acid,  and  Sulphuric  Acid  are 
the  three  acids  with  which  most  of  the  bases  that  compose 
the  earth's  crust  have  been  combined.  With  these  we  may 
connect  the  water  which,  besides  merely  percolating  through 
rocks,  or  existing  inclosed  in  the  vesicles  of  minerals,  has 
been  chemically  absorbed  in  the  process  of  hydration,  and 
which  thus  constitutes  more  than  10  or  even  20  per  cent  of 
some  rocks  (gypsum). 

Chemical  analysis  has  revealed  the  numerous  combina- 
tions in  which  the  elements  are  united  to  form  minerals  and 
rocks.  Considerable  additional  light  has  been  thrown  on 
the  subject  by  chemical  synthesis,  that  is,  by  artificially 
producing  the  minerals  and  rocks  which  are  found  in  na- 
ture. The  experiments  have  been  varied  indefinitely  so  as 
to  imitate  as  far  as  possible  the  natural  conditions  of  produc- 
tion. Further  reference  to  this  subject  will  be  found  on 
pp.  161,  505  et  seq. 

Although  every  mineral  may  be  made  to  yield  data  of 
more  or  less  geological  significance,  only  those  minerals 
need  be  referred  to  here  which  enter  as  chief  ingredients 
into  the  composition  of  rock-masses,  or  which  are  of  fre- 
quent occurrence  as  accessories,  and  special  note  may  be 
taken  of  those  of  their  characters  which  are  of  main  interest 


GEOGNOSY  119 

from  a  geological  point  of  view,  such  as  their  modes  of  oc- 
currence in  relation  to  the  genesis  of  rocks,  and  their  weath- 
ering as  indicative  of  the  nature  of  rock-decomposition. 

§  ii.  Rock-forming  Minerals 

Minerals,  as  constituents  of  rocks,  occur  in  four  condi- 
tions, according  to  the  circumstances  under  which  they 
have  been  produced. 

(1.)  Crystalline,  as  (a)  more  or  less  regularly  defined 
crystals,  which,  exhibiting  the  outlines  proper  to  the 
mineral  to  which  they  belong,  are  said  to  be  idiomorphic; 
(6)  amorphous  granules,  aggregations  or  crystalloids,  having 
an  internal  crystalline  structure,  in  most  cases  easily  recog- 
nizable with  polarized  light,  as  in  the  quartz  of  granite, 
and  an  external  form  which  has  been  determined  by  con- 
tact with  the  adjacent  mineral  particles;  such  crystalline 
bodies  which  do  not  exhibit  their  proper  crystalline  out- 
lines are  said  to  be  allotriomorphic;  (c)  ''crystallites"  or 
"microlites,"  incipient  forms  of  crystallization,  which  are 
described  on  p.  206.  The  crystalline  condition  may  arise 
from  igneous  fusion,  aqueous  solution,  or  sublimation.6 

(2.)  Glassy  or  vitreous,  as  a  natural  glass,  usually  in- 
cluding either  crystals  or  crystallites,  or  both.  Minerals 
have  assumed  this  condition  from  a  state  of  fusion,  also 
from  solution.  The  glass  may  consist  of  several  minerals 
fused  into  one  homogeneous  substance.  Where  it  has  as- 
sumed a  lithoid  or  stony  structure,  these  component  min- 
erals crystallize  out  of  the  glassy  magma,  and  may  be 
recognized  in  various  stages  of  growth  (postea,  pp.  194- 
214). 

6  For  the  microscopic  characters  of  minerals  and  rocks,  see  p.  192. 


120  TEXT-BOOK    OF   GEOLOGY 

(3.)  Colloid,  as  a  jelly-like  though  stony  substance,  de- 
posited from  aqueous  solution.  The  most  abundant  mineral 
in  nature  which  takes  the  colloid  form  is  silica.  Opal  is 
a  hardened  colloidal  condition  of  this  substance.  Chalced- 
ony, doubtless  originally  colloidal  silica,  now  unites  the 
characters  of  quartz  and  opal,  being  only  partially  soluble 
in  caustic  potash  and  partially  converted  into  a  finely 
fibrous,  doubly-refracting  substance. 

(4.)  Amorphous,  having  no  crystalline  structure  or  form, 
and  occurring  in  indefinite  masses,  granules,  streaks,  tufts, 
stainings,  or  other  irregular  modes  of  occurrence. 

A  mineral  which  has  replaced  another  and  has  assumed 
the  external  form  of  the  mineral  so  replaced,  is  termed  a 
Pseudomorph.  A  mineral  which  incloses  another  has  been 
called  a  Perimorph]  one  inclosed  within  another,  an 
Endomorph. 

Essential  or  accessory,  original  or  secondary  'minerals. — 
A  mineral  is  an  essential  ingredient  when  its  absence  would 
so  alter  the  character  of  a  rock  as  to  make  it  something 
fundamentally  different.  The  quartz  of  granite,  for  exam- 
ple, is  an  essential  constituent  of  that  rock,  the  removal  of 
which  would  alter  the  petrographical  species.  A  mineral 
is  said  to  be  accessory  when  its  absence  would  not  change 
the  essential  character  of  the  rock.  All  essential  minerals 
are  original  constituents  of  a  rock,  but  all  the  original  con- 
stituents are  not  essential.  In  granite,  such  minerals  as 
topaz,  beryl,  and  sphene  often  occur  under  circumstances 
which  show  that  they  crystallized  out  of  the  original  magma 
of  the  rock.  But  they  form  so  trifling  a  proportion  in  the 
total  mass,  and  their  absence  would  so  little  affect  the  gen- 
eral character  of  that  mass,  that  they  are  regarded  as  ac- 
cessory, though  undoubtedly  original  and  often  important 


GEOGNOSY  121 

ingredients.6  Again,  in  rocks  of  eruptive  origin,  the  essen- 
tial ingredients  cannot  be  traced  back  further  than  the 
eruption  of  the  mass  containing  them.  They  are  not  only 
original,  as  constituents  of  the  lava,  but  are  themselves 
original  and  non-derivative  minerals,  produced  directly 
from  the  crystallization  of  molten  minerals  ejected  from 
beneath  the  earth's  crust,  though,  as  Michel-LeVy  has 
shown,  the  debris  of  older  minerals  may  sometimes  be 
traced  amid  the  later  crystals  of  massive  rocks.7  In  rocks 
of  aqueous  origin,  however,  there  are  many,  such  as  con- 
glomerates and  sandstones,  where  the  component  minerals, 
though  original  ingredients  of  the  rocks,  are  evidently  of 
derivative  origin.  The  little  quartz-granules  of  a  sand- 
stone have  formed  part  of  the  rock  ever  since  it  was  ac- 
cumulated, and  are  its  essential  constituents.  Yet  each 
of  these  once  formed  part  of  some  older  rock,  the  destruc- 
tion of  which  yielded  materials  for  the  production  of  the 
sandstone.  The  minute  crystals  of  zircon,  rutile,  tourma- 
line and  other  minerals  so  often  found  in  sands,  clays, 
sandstones,  shales  and  other  sedimentary  deposits,  have 
been  derived  from  the  degradation  of  older  crystalline 
rocks. 

The  same  mineral  may  occur  both  as  an  original  and 
as  a  secondary  constituent.  Quartz,  for  example,  appears 
everywhere  in  both  conditions;  indeed,  it  may  sometimes 
be  found  in  a  twofold  form  even  in  the  same  rock,  though 

6  Some  of  the  "accessory"  minerals  may  be  of  great  importance  as  indicative 
of  the  conditions  under  which  the  rock  was  formed. 

1  Bull.  Soc.  Geol.  France,  3d  ser.  iii.  199.  See  also  Fouque  and  Michel- 
LeVy,  "Mineralogie  Micrographique, "  p.  189.  Some  eruptive  rocks  abound  in 
corroded  or  somewhat,  rounded  or  broken  crystals  which  obviously  have  belonged 
to  some  previous  state  of  consolidation.  Such  crystals,  which  are  obviously 
moie  undent  than  those  forming  the  general  mass  of  the  rock,  have  been  called 
alloyenic,  while  those  which  belong  to  the  time  of  formation  of  the  rock,  or  to 
some  subsequent  change  within  the  rock,  are  known  as  authigenic. 
GEOLOGY— Vol.  XXIX— 6 


122  TEXT-BOOK   OF   GEOLOGY 

there  is  then  usually  some  difference  between  the  orig- 
inal and  secondary  quartz.  A  quartz-felsite,  for  instance, 
abounds  in  original  little  kernels,  or  in  double  pyramids 
of  the  mineral,  often  inclosing  fluid  cavities,  while  the 
secondary  or  accidental  forms  usually  occur  in  veins,  retic- 
ulations, or  other  irregular  aggregates. 

Accessory  minerals  frequently  occur  in  cavities  where 
they  have  had  some  room  to  crystallize  out  from  the  general 
mass.  The  "drusy"  cavities,  or  open  spaces  lined  with  well- 
developed  crystals,  found  in  some  granites  are  good  exam- 
ples, for  it  is  there  that  the  non-essential  minerals  are 
chiefly  to  be  recognized.  The  veins  of  segregation  found 
in  many  crystalline  rocks,  particularly  in  those  of  the 
granite  series,  are  further  illustrations  of  the  original  sepa- 
ration of  mineral  ingredients  from  the  general  magma  of 
a  rock  (see  Book  IV.  Part  VII.  §  3). 

In  some  cases  minerals  assume  a  concretionary  shape, 
which  may  be  observed  chiefly  though  not  entirely  in  rocks 
formed  in  water.  Some  minerals  are  particularly  prone 
to  occur  in  concretions.  Siderite  (ferrous  carbonate)  is  to 
be  found  in  abundant  nodules,  mixed  with  clay  and  or- 
ganic matter  among  consolidated  muddy  deposits.  Calcite 
(calcium-carbonate)  is  likewise  abundantly  concretionary. 
Silica  in  the  forms  of  chert  and  flint  appears  in  irregular 
concretions,  in  calcareous  formations,  composed  mainly  of 
the  remains  of  marine  organisms. 

Secondary  minerals  have  been  developed  as  the  result 
of  subsequent  changes  in  rocks,  and  are  almost  invariably 
due  to  the  chemical  action  of  percolating  water,  either  from 
above  or  from  below.  Occurring  under  circumstances  in 
which  such  water  could  act  with  effect,  they  are  found 
in  cracks,  joints,  fissures  and  other  divisional  planes  and 


GEOGNOSY  123 

cavities  of  rocks,  especially  in  the  minute  interspaces  be- 
tween the  component  grains  or  minerals.  Subterranean 
channels,  frequently  several  feet  or  even  yards  wide, 
have  been  gradually  filled  up  by  the  deposit  of  mineral 
matter  on  their  sides  (see  the  Section  on  Mineral  Veins). 
The  cavities  formed  by  expanding  steam  in  ancient  lavas 
(amygdaloids)  have  offered  abundant  opportunities  for  de- 
posits of  this  kind,  and  have  accordingly  been  in  large 
measure  occupied  by  secondary  minerals  (amygdales),  as 
calcite,  chalcedony,  quartz  and  zeolites. 

In  the  subjoined  list  of  the  more  important  rock- forming 
minerals,  attention  is  drawn  mainly  to  those  features  that 
are  of  geological  importance;  the  physical,  chemical  and 
microscopic  characters  of  these  minerals  will  be  found  in 
a  text-book  of  mineralogy  or  petrography.  Eeference  is 
therefore  made  here  to  features  of  more  special  signifi- 
cance to  the  geologist,  such  as  modes  of  occurrence, 
whether  original  or  secondary;  modes  of  origin,  whether 
igneous,  aqueous,  or  organic;  pseudomorphs,  that  is,  the 
various  minerals  which  any  given  mineral  has  replaced, 
while  retaining  their  external  forms,  and  likewise  those 
which  are  found  to  have  supplanted  the  mineral  in  ques- 
tion while  in  the  same  way  retaining  its  form — a  valuable 
clew  to  the  internal  chemical  changes  which  rocks  undergo 
from  the  action  of  percolating  water  (Book  III.  Part  H. 
Section  ii.  §§  1  and  2);  and  lastly,  characteristics  or  peculi- 
arities of  weathering,  where  any  such  exist  that  deserve 
special  mention. 

1.  NATIVE  ELEMENTS  are  comparatively  of  rare  occur- 
rence, and  only  two  of  them,  Carbon  and  Sulphur,  occa- 
sionally play  the  part  of  noteworthy  essential  and  accessory 
constituents  of  rocks.  A  few  of  the  native  metals,  more 


124  TEXT-BOOK    OF   GEOLOGY 

especially  copper  and  gold,  now  and  then  appear  in  suffi- 
cient quantity  to  constitute  commercially  important  ingredi- 
ents of  veins  and  rock-masses. 

Graphite  is  found  chiefly  in  ancient  crystalline  rocks,  as 
gneiss,  mica-schist,  granite,  etc. ;  some  of  the  Laurentian 
limestones  of  Canada  being  so  full  of  the  diffused  mineral 
as  to  be  profitably  worked  for  it;  in  rare  instances  coal  has 
been  observed  changed  into  it  by  intrusive  basalt  (Ayrshire). 
In  some  cases  graphite  results  from  the  alteration  of  im- 
bedded organic  matter,  especially  remains  of  plants;  but  its 
presence,  and  that  of  diamond,  among  ancient  crystalline 
rocks  and  in  meteorites  can  hardly  be  thus  accounted  for. 
Occasionally  it  is  observed  as  a  pseudomorph  after  calcite 
and  pyrites,  and  sometimes  inclosing  sphene  and  other 
minerals.8 

Sulphur  occurs  1st,  as  a  product  of  volcanic  action  in  the 
vents  and  fissures  of  active  and  dormant  cones.  Volcanic 
sulphur  is  formed  from  the  oxidation  of  the  sulphuretted 
hydrogen,  so  copiously  emitted  with  the  steam  that  issues 
from  volcanic  vents,  as  at  the  Solfatara,  near  Naples.  It 
may  also  be  produced  by  the  mutual  decomposition  of  the 
same  gas  and  anhydrous  sulphuric  acid.  2d,  in  beds  and 
layers,  or  diffused  particles,  resulting  from  the  alteration 
of  previous  minerals,  particularly  sulphates,  or  from  deposit 
in  water  through  decomposition  of  sulphuretted  hydrogen. 
The  frequent  crystallization  of  sulphur  shows  that  the 
mineral  must  have  been  formed  at  ordinary  temperatures, 
for  its  natural  crystals  melt  at  238-1°  Fahr.  Its  formation 
may  be  observed 'in  progress  at  many  sulphureous  springs, 
where  it  falls  to  the  bottom  as  a  pale  mud  through  the  oxi- 
dation of  the  sulphuretted  hydrogen  in  the  water.  It  occurs 
in  Sicily,  Spain  and  elsewhere,  in  beds  of  bituminous  lime- 
stone and  gypsum.  These  strata,  sometimes  full  of  remains 
of  fresh-water  shells  and  plants,  are  interlaminated  with 
sulphur,  the  very  shells  being  not  infrequently  replaced 
by  this  mineral.  Here  the  presence  of  the  sulphur  may  be 
traced  to  the  reduction  of  the  calcium-sulphate  to  the  state 
of  sulphide,  through  the  action  of  the  decomposing  organic 
matter,  and  the  subsequent  production  and  decomposition 
of  sulphuretted  hydrogen,  with  consequent  liberation  of 
sulphur."  The  sulphur  deposits  of  Sicily  furnish  an  excel- 

8  Vom  Rath.  Sitzungsber.  Wien.  Akad.  x.  p.  67 ;  Sullivan  in  Jukes'  "Manual 
of  Geology,"  3d  edit.  (1872),  p.  56. 

9  Braun,  Bull.  Soc.  Geol.  France,  1st  ser.  xil  p.  171. 


GEOGNOSY  125 

lent  illustration  of  the  alternate  deposit  of  sulphur  and 
limestone.  They  consist  mainly  of  a  marly  limestone, 
through  which  the  sulphur  is  partly  disseminated  and 
partly  interstratified  in  thin  laminae  and  thicker  layers, 
some  of  which  are  occasionally  28  feet  deep.  Below  these 
deposits  lie  older  Tertiary  gypseous  formations,  the  decom- 
position of  which  has  probably  produced  the  deposits  of 
sulphur  in  the  overlying  more  recent  lake  basins.10  The 
weathering  of  sulphur  is  exemplified  on  a  considerable 
scale  at  these  Sicilian  deposits.  The  mineral,  in  presence 
of  limestone,  oxygen  and  moisture,  becomes  sulphuric  acid, 
which,  combining  with  the  limestone,  forms  gypsum,  a  cu- 
rious return  to  what  was  probably  the  original  substance 
from  the  decomposition  of  which  the  sulphur  was  derived. 
Hence  the  site  of  the  outcrop  of  the  sulphur  beds  is  marked 
at  the  surface  by  a  white  earthy  rock,  or  borscale,  which  is 
regarded  by  the  miners  in  Sicily  to  be  a  sure  indication 
of  sulphur  underneath,  as  the  gossan  of  Cornwall  is  indic- 
ative of  underlying  metalliferous  veins.11 

Iron,  the  most  important  of  all  the  metals,  is  found  only 
sparingly  in  the  native  state,  in  blocks  which  have  fallen  as 
meteorites,  also  in  grains  or  dust  inclosed  in  hailstones,  in 
snow  of  the  Alps,  Sweden  and  Siberia,  in  the  mud  of  the 
ocean-floor  at  remote  distances  from  land,  and  in  some  erup- 
tive rocks.  There  can  be  no  doubt  that  a  small  but  con- 
stant supply  of  native  iron  (cosmic  dust)  is  falling  upon  the 
earth's  surface  from  outside  the  terrestrial  atmosphere.1* 
This  iron  is  alloyed  with  nickel,  and  contains  small  quan- 
tities of  cobalt,  copper  and  other  ingredients.  Dr.  An- 
drews, however,  showed  in  1852  that  native  iron,  in  minute 
spicules  or  granules,  exists  in  some  basalts  'and  other  vol- 
canic rocks13  and  Mr.  J.  Y.  Buchanan  has  detected  it  in  ap- 

>o  Memorie  del  R.  Comitato  Geologico  d'ltalia,  i.  (1871). 

11  Journ.  Soc.  Arts,  1873,  p.  170.     B.  Ledoux,  Ann.  dea  Mines,  7me.  ser. 
vii.  p.  1.     The  Sicilian  sulphur  beds  belong  to  the  Oeningen  stage  of  the  Upper 
Tertiary  deposits.     They  contain  numerous  plants  and  some  insects.     H.  T. 
Geyler,  Palseontographica,  xxiii.  Lief.  9,  p.  317.     Von  Lasaulx,  Neues  Jahrb. 
1879,  p.  490. 

12  See  Bhrenberg,   Frorieps  Notizen,    Feb.    1846;    Nordenskiold,   Comptes 


reudus,  Ixxvii.  p.  463,  Ixxviii.  p.  236.     Tissandier,  op.  cit.  Ixxviii.  p.  821,  Ixxx. 

&58,  Ixxxi.  p.  576.     See  Ixxv.  (1872)  p.  683.     Yung,  Bull.  Soc.  Vaudoise  Sci. 
at.  (1876),  xiv.  p.  493.     Ranyard,  Monthly  Not.  Roy.  Astron.  Soc.  xxxix. 


(1879)  p.  161.  S.  L.  Phipson,  Comptes  rend.  Ixxxiii.  p.  364.  A  Committee  of 
the  British  Association  was  appointed  in  1880  to  investigate  the  subject  of  cosmic 
dust.  See  its  reports  for  1881-83. 

13  Brit.  Assoc.  Rep.  1852,  postea,  p.    7(38. 


126  TEXT-BOOK   OF  GEOLOGY 

preciable  quantity  in  the  gabbro  of  the  west  of  Scotland. 
It  occurs  also  in  the  basalts  of  Bohemia  and  Greenland.14 

In  the  great  majority  of  cases  the  OXIDES  occur  com- 
bined with  some  acid.  A  few  uncombined  take  a  promi- 
nent place  as  essential  constituents  or  frequent  ingredients 
of  rocks,  especially  the  oxides  of  silicon  and  iron. 

2.  SILICA  (SiO9)  is  found  in  three  chief  forms,  Quartz, 
Tridymite,  and  Opal. 

Quartz  is  abundant  as  (1)  an  essential  constituent  of  rocks, 
as  in  granite,  gneiss,  mica-schist,  rhyolite  (quartz-trachyte), 
quartz-porphyry,  sandstone;  (2)  a'  secondary  ingredient, 
wholly  or  partially  filling  veins,  joints,  cracks,  and  cavi- 
ties. It  has  been  produced  from  (a)  igneous  action,  as  in 
volcanic  rocks;  (&)  aquo-igneous  or  plutonic  action,  as  in 
granites,  gneisses,  etc.;  (c)  solution  in  water,  as  where  it 
lines  cavities  or  replaces  other  minerals.  The  last  mode 
of  formation  is  that  of  the  crystallized  quartz  and  chalced- 
ony found  as  secondary  ingredients  in  rocks. 

The  study  of  the  endomorphs  and  pseudomorphs  of 
quartz  is  of  great  importance  in  the  investigation  of  the 
history  of  rocks.  No  mineral  is  so  conspicuous  for  the 
variety  of  other  minerals  inclosed  within  it.  In  some  sec- 
ondary quartz-crystals,  each  prism  forms  a  small  mineralog- 
ical  cabinet  inclosing  a  dozen  or  more  distinct  minerals,  as 
rutile,  haematite,  limonite,  pyrites,  chlorite,  and  many 
others.16  Quartz  may  be  observed  replacing  calcite,  arago- 
nite,  siderite,  gypsum,  rock-salt,  haematite,  etc.  This  facil- 
ity of  replacement  makes  silica  one  of  the  most  valuable 

14  Nordenskiold  describes  fifteen  blocks  of  iron  on  the  island  of  Disco,  Green- 
land, the  weight  of  the  two  largest  being  21,000  and  8000  kilogrammes  (20  and 
8  tons,  respectively).  He  observed  that  at  the  same  locality,  the  underlying 
basalt  contains  lenticular  and  disk-shaped  blocks  of  precisely  similar  iron,  and 
inferred  that  the  whole  of  the  blocks  may  belong  to  a  meteoric  shower  which 
fell  during  the  time  (Tertiary)  when  the  basalt  was  poured  out  at  .the  surface. 
He  dismissed  the  suggestion  that  the  iron  could  possibly  be  of  telluric  origin 
(Geol.  Mag.  ix.  (1872)  p.  462).  But  the  microscope  reveals  in  this  basalt  the 
presence  of  minute  particles  of  native  iron  which,  associated  with  viridite,  are 
molded  round  the  crystals  of  labradorite  and  augite  (Fouque"  and  Michel-Levy, 
op.  cit.  p.  443).  Steenstrup,  Daubr6e,  and  others  appear  therefore  to  be  justi- 
fied in  regarding  this  iron  as  derived  from  the  inner  metallic  portions  of  the 
globe,  which  lie  at  depths  inaccessible  to  our  observations,  but  from  which  the 
vast  Greenland  basalt  eruptions  have  brought  up  traces  to  the  surface  (K.  J.  T. 
Steenstrup,  Vid.  Medd.  Nat.  Foren.  Copenhagen  (1875)  Nos.  16-19,  p.  284; 
Zeitsch.  Deutsch.  Geol.  Ges.  xxviii.  (1876)  p.  225 ;  Mineralog.  Mag.  July,  1884. 
P.  Wohler,  Neues  Jahrb.  1879,  p.  832.  Daubree,  Discours  Acad.  Sci.  1  March, 
1880,  p.  17.  W.  Flight,  Geol.  Mag.  ii.  (2d  ser.)  p.  152. 

16  See  Sullivan,  in  Jukes'  "Manual  of  Geology,"  3d.  edit.  (1872),  p.  61. 


GEOGNOSY  127 

petrifying  agents  in  nature.  Organic  bodies  which  have 
been  silicified  retain,  often  with  the  utmost  perfection,  their 
minutest  and  most  delicate  structures. 

Quartz  may  usually  be  identified  by  its  external  charac- 
ters, and  especially  oy  its  vitreous  lustre  and  hardness. 
When  in  the  form  of  minute  blebs  or  crystals,  it  may  be 
recognized  in  many  rocks  with  a  good  lens.  Under  the 
microscope,  it  presents  a  characteristic  brilliant  chromatic 
polarization,  and  in  convergent  light  gives  a  black  cross. 
Where  it  is  an  original  and  essential  constituent  of  a  rock, 
quartz  very  commonly  contains  minute  rounded  or  irregular 
cavities  or  pores,  partially  filled  with  liquid.  So  minute  are 
these  cavities  that  a  thousand  millions  of  them  may,  when 
they  are  closely  aggregated,  lie  within  a  cubic  inch.  The 
liquid  is  chiefly  water,  not  uncommonly  containing  sodium 
chloride  or  other  salt,  sometimes  liquid  carbon-dioxide  and 
hydrocarbons.1'  Chalcedony  exhibits  under  the  microscope 
a  minute  radial  fibrous  structure. 

Bock-crystal  and  crystalline  quartz  resist  atmospheric 
weathering  with  great  persistence.  Hence  the  quartz-grains 
may  usually  be  easily  discovered  in  the  weathered  crust  of 
a  quartziferous  igneous  rock.  But  corroded  quartz-crystals 
have  been  observed  in  exposed  mountainous  situations,  with 
their  edges  rounded  and  eaten  away.17  The  chalcedonic  and 
more  or  less  soluble  forms  of  silica  are  more  easily  affected. 
Flint  and  many  forms  of  colored  chalcedony  weather  with  a 
white  crust.  6ut  it  is  chiefly  from  the  weathering  of  sili- 
cates (especially  through  the  action  of  organic  acids)  that  the 
soluble  silica  of  natural  waters  is  derived.  (Book  III.  Part 
II.  Section  ii.  §  7.) 

Tridymite  has  been  met  with  chiefly  among  volcanic  rocks 
(trachytes,  andesites,  etc.),  both  as  an  abundant  constituent 
of  those  which  have  been  poured  out  in  the  form  of  lava, 
and  also  in  ejected  blocks  (Vesuvius).18 

Opal,  a  hydrous  condition  of  silica  formed  from  solution 
in  water,  is  usually  disseminated  in  veins  and  nests  through 
rocks.  Semi-opal  occasionally  replaces  the  original  sub- 

'•  See  Brewster,  Trans.  Roy.  Soc.  Edin.  x.  p.  1.  Sorby,  Quart.  Journ.  Geol. 
Soc.  xiv.  p.  463.  Proc.  Roy.  Soc.  xv.  p.  153;  xvii.  p.  299.  Zirkel,  "Mikro- 
skopische  Beschaffenheit  der  Mineralien  und  Gesteine,"  p.  39.  Rosenbusch, 
"Mikroskopische  Physiographic, "  i.  p.  30.  Hartley,  Journ.  Chem.  Soc.  Feb- 
ruary, 1876.  The  occurrence  of  fluid-cavities  in  the  crystals  of  rocks  is  more 
fully  described  in  Part  II.  §  iv.  of  this  Book. 

17  Roth,  Chern.  Geol.  i.  p.  94. 

18  Vom  Rath,  Z.  Deutsch.  Geol.  Ges.  xxv.  p.  236,  1873. 


128  TEXT-BOOK    OF   GEOLOGY 

stance  of  fossil  wood  (wood-opal).  Several  forms  of  opal 
are  deposited  by  geysers,  and  are  known  under  the  general 
appellation  of  sinters.  Closely  allied  to  the  opals  are  the 
forms  in  which  hydrous  (soluble)  silica  appears  in  the  or- 
ganic world,  where  it  constitutes  the  frustules  of  diatoms, 
the  skeletons  of  radiolaria,  etc.  Tripoli  powder  (Kiesel- 
guhr),  randanite,  and  other  similar  earths,  are  composed 
mainly  or  wholly  of  the  remains  of  diatoms,  etc. 

Corundum,  aluminium-oxide,  is  found  in  crystalline  rocks, 
particularly  in  certain  serpentines  and  schists,  gneiss,  gran- 
ite, dolomite,  and  rocks  of  the  metamorphic  series. 

3.  IRON  OXIDES. — Four  minerals,  composed  mainly  of 
iron  oxides,  occur  abundantly  as  essential  and  accessory  in- 
gredients of  r.ocks.  Haematite,  Limonite,  Magnetite,  and 
Titanic  iron. 

Hxmatite  (Fer  oligiste,  Eotheisen,  Eisenglanz,  Fe,O3= 
Fe70,  O30)  in  the  crystallized  form  occurs  in  veins,  as  well 
as  lining  cavities  and  fissures  of  rocks.  The  fibrous  and 
more  common  form  (which  often  has  portions  of  its  mass 
passing  into  the  crystallized  condition)  lies  likewise  in 
strings  or  veins;  also  in  cavities,  which,  when  of  large 
size,  have  given  opportunity  for  the  deposit  of  great  masses 
of  haematite,  as  in  cavernous  limestones  (Westmoreland). 
It  occurs  with  other  ores  and  minerals  as  an  abundant  com- 
ponent of  mineral  veins,  likewise  in  beds  interstratified  with 
sedimentary  or  schistose  rocks.  Scales  and  specks  of  opaque 
or  clear  bright  red  haematite,  of  frequent  occurrence  in  the 
crystals  of  rocks,  give  them  a  reddisn  color  or  peculiar  lustre 
(perthite,  stilbite).  Haematite  appears  abundantly  as  a  prod- 
uct of  sublimation  in  clefts  of  volcanic  cones  and  lava 
streams.  It  is  probably  in  most  cases  a  deposition  from 
water,  resulting  from  the  alteration  of  some  previous  solu- 
ble combination  of  the  metal,  such  as  the  oxidation  of  the 
sulphate,  and  occurs  in  veins  and  beds,  and  as  the  earthy 
pigment  that  gives  a  red  color  to  sandstones,  clays  and  other 
rocks.  It  is  found  pseudomorphous  after  ferrous  carbonate, 
and  this  has  probably  been  the  origin  of  beds  of  red  ochre 
occasionally  intercalated  among  stratified  rocks.  It  likewise 
replaces  calcite,  dolomite,  quartz,  barytes,  pyrites,  magne- 
tite, rock-salt,  fluor-spar,  etc. 

Ljmonite  (Brown  iron -ore,  2Fe2O8+3H,O  =  FeA  85-56, 
H2O  14-44)  occurs  in  beds  among  stratified  formations,  and 
may  be  seen  in  the  course  of  deposit,  through  the  action  of 
organic  acids,  on  marsh- land  (bog-iron-ore)  and  lake-bottoms. 
(Book  IV.  Part  II.  Section  iii.)  In  the  form  of  yellow 


GEOGNOSY  129 

ochre,  it  is  precipitated  from  the  waters  of  chalybeate 
springs  containing  green  vitriol  derived  from  the  oxidation 
pi  iron-sulphides."  It  is  a  common  decomposition  product 
in  rocks  containing  iron  among  their  constituents.  It  is 
thus  always  a  secondary  or  derivative  substance,  resulting 
from  chemical  alteration.  It  is  the  usual  pigment  which 
gives  tints  of  yellow,  orange  and  brown  to  rocks.  The 
pseudomorphous  forms  of  limonite  show  to  what  a  large  ex- 
tent combinations  of  iron  are  carried  in  solution  through 
rocks.  The  mineral  has  been  found  replacing  calcite,  sid- 
erite,  dolomite,  haematite,  magnetite,  pyrite,  marcasite,  ga- 
lena, blende,  gypsum,  barytes,  fluor-spar,  pyroxene,  quartz, 
garnet,  beryl,  etc. 

Magnetite  (Fer  oxydule*,  Magneteisen,  Fe3O<)  occurs  abun- 
dantly in  some  schists,  in  scattered  octohedral  crystals;  in 
crystalline  massive  rocks  like  granite,  in  diffused  grains  or 
minute  crystals;  among  some  schists  and  gneisses  (Norway 
and  the  Eastern  States  of  North  America),  in  massive  beds; 
in  basalt  and  other  volcanic  rocks,  as  an  essential  constitu- 
ent, in  minute  octohedral  crystals,  or  in  granules  or  crystal- 
lites. It  is  likewise  found  as  a  pseudomorphous  secondary 
product,  resulting  from  the  alteration  of  some  previous  min- 
eral, as  olivine,  hematite,  pyrite,  quartz,  hornblende,  au- 
gite,  garnet  and  sphene.  It  occurs  with  haematite,  etc.,  as  a 
product  of  sublimation  at  volcanic  foci,  where  chlorides  of 
the  metals  in  presence  of  steam  are  resolved  into  hydro- 
chloric acid  and  anhydrous  oxides.  It  may  thus  result 
from  either  aqueous  or  igneous  operations.  It  is  liable  to 
weather  by  the  reducing  effects  of  decomposing  organic  mat- 
ter, whereby  it  becomes  a  carbonate,  and  then  by  exposure 
passes  into  the  hydrous  or  anhydrous  peroxide.  The  mag- 
netite grains  of  basalt-rocks  are  very  generally  oxidized  at 
the  surface,  and  sometimes  even  for  some  depth  inward. 

Titanic  Iron  (Titaniferous  Iron,  Menaccanite,  Ilmenite,  Fer 
titane,  Titaneisen  (FeTi)9O3)  occurs  in  scattered  grains,  plates 
and  crystals  as  an  abundant  constituent  of  many  crystalline 
rocks  (basalt-rocks,  diabase,  gabbro  and  other  igneous 
masses);  also  in  veins  or  beds  in  syenite,  serpentine  and 
metamorphic  rocks ; '•  scarcely  to  be  distinguished  from 
magnetite  when  seen  in  small  particles  under  the  micro- 
scope, but  possessing  a  brown  semi-metallic  lustre  with  re- 

"  Sullivan,  Jukes'  "Manual  of  Geology,"  p.  63. 

20  Some  of  the  Canadian  masses  of  this  mineral  are  90  feet  thick  and  manj 
yards  in  length. 


130  TEXT-BOOK    OF   GEOLOGY 

fleeted  light;  resists  corrosion  by  acids  when  the  powder  of 
a  rock  containing  it  is  exposed  to  their  action,  while  mag- 
netite is  attacked  and  dissolved.  Titanic  iron  frequently 
resists  weathering,  so  that  its  black  glossy  granules  project 
from  a  weathered  surface  of  rock.  In  other  cases,  it  is  de- 
composed either  by  oxidation  of  its  protoxide,  when  the 
usual  brown  or  yellowish  color  of  the  hydrous  ferric  oxide 
appears,  or  by  removal  of  the  iron.  The  latter  is  believed 
to  oe  the  origin  of  a  peculiar  milky  white  opaque  substance, 
frequently  to  be  observed  under  the  microscope,  surround- 
ing and  even  replacing  crystals  of  titanic  iron,  and  named 
Leucoxene  by  Grumbel.ai  In  other  cases  the  decomposition 
has  resulted  in  the  production  of  sphene. 

Chromite  (FeCr,O4)  occurs  in  black  opaque  grains  and 
crystals  not  infrequently  in  altered  oHvine-rocks. 

Spinels,  a  group  of  minerals,  may  be  taken  here.  They 
are  closely  related  to  each  other,  having  cubic  forms  and 
varying  in  composition  from  magnetite  (see  above)  at  the 
one  end  to  spinet  (MgAl?O4)  at  the  other.  They  are  not  in- 
frequent as  minute  grains  or  crystals  in  some  igneous  and 
metamorphic  rocks.  Between  magnetite  and  spinel  come 
intermediate  varieties,  as  chromite  (see  above),  Picotite,  Her- 
cynite  and  Pleonaste. 

4.  MANGANESE  OXIDES  are  frequently  associated  with 
those  of  iron  in  ordinary  rock-forming  minerals,  but  in  such 
minute  proportions  as  to  have  been  generally  neglected  in 
analyses.     Their  presence  in  the  rocks  of  a  district  is  some- 
times shown  by  deposits  of  the  hydrous  oxide  in  the  forms 
of  Psilomelane  (H2MnO4+H,O)  and  Wad  (MnO8-f-MnO+ 
H»O).     These  deposits  sometimes  take  place   as   black  or 
dark  brown  branching,  plant-like  or  dendritic  impressions 
between  the  divisional  planes  of  close-grained  rocKs  (lime- 
stone, felsite,  etc.),  sometimes  as  accumulations  of  a  black 
or  brown  earthy  substance  in  hollows  of  rocks,  occasionally 
as  deposits  in  marshy  places,  like  those  of  bog-iron-ore,  and 
abundantly  on  someparts  of  the  sea-floor.     (See  p.  769.) 

5.  SILICATES. — These  embrace  by  far  the   largest   and 
most  important   series   of    rock-forming    minerals.      Their 
chief  groups  are  the  anhydrous  aluminous  and  magnesian 
silicates   embracing    the    Felspars,    Hornblendes,    Augites, 

S1  "Die  Palaolitische  Eruptivgesteine  dee  Fichtelgebirges,"  1874,  p.  29.  See 
Rosenbusch,  Mik.  Physiog.  ii.  p.  336.  De  la  Vallee  Pouesin  and  Renard,  Mem. 
Gouronnees  Acad.  Roy.  de  Belgique,  1876,  xl.  Plate  vi.  pp.  34  and  35.  Fouque" 
and  Michel-Levy,  "Mineralogie  Micrograph,"  p.  426.  See  postea,  p.  1040. 


GEOGNOSY  131 

Micas,   etc.,   and  the  hydrous  silicates  which    include  the 
Zeolites,  Clays,  talc,  chlorite,  serpentine,  etc. 

The  family  of  the  Felspars  forms  one  of  the  most  im- 

Krtant  of  all  the  constituents  of  rocks,  seeing  that  its  inem- 
rs  constitute  by  much  the  largest  portion  of  the  plutonic 
and  volcanic  rocks,  are  abundantly  present  among  many 
crystalline  schists,  and  by  their  decay  nave  supplied  a  great 
part  of  the  clay  out  of  which  argillaceous  sedimentary  for- 
mations have  been  constructed. 

The  felspars  are  usually  divided  into  two  series.  1st, 
The  orthoclastic  or  monociinic  felspars,  consisting  of  two 
species  or  varieties,  Orthoclase  and  Sanidine;  and,  2d,  The 
plagioclastic  or  triclinic  felspars,  among  which,  as  constitu- 
ents of  rocks,  may  be  mentioned  the  species  albite,  anorthite, 
oligoclase,  andesine,  labradorite,  and  microcline. 

Orthoclase  (K,O  16-89,  A12O3  1843,  SiO,  64-68)  occurs 
abundantly  as  an  original  constituent  of  many  crystalline 
rocks  (granite,  syenite,  felsite,  gneiss,  etc.),  likewise  in  cavi- 
ties and  veinings  in  which  it  has  segregated  from  the  sur- 
rounding mass  (pegmatite);  seldom  found  in  unaltered 
sedimentary  rocks  except  in  fragments  derived  from  old 
crystalline  masses;  generally  associated  with  quartz,  and 
often  with  hornblende,  while  the  felspars  less  rich  in  silica 
more  rarely  accompany  free  quartz.  It  is  an  original  con- 
stituent of  plutonic  and  old  volcanic  rocks  (granite,  felsite, 
etc.),  and  of  gneiss  and  various  schists.  A  few  examples 
have  been  noticed  where  it  has  replaced  other  minerals 
(prehnite,  analcime,  laumontite).  Under  the  microscope  it 
is  recognizable  from  quartz  by  its  characteristic  rectangular 
forms,  cleavage,  twinning,  angle  of  extinction,  turbidity, 
and  frequent  alteration."  Orthoclase  weathers  on  the  whole 
with  comparative  rapidity,  though  durable  varieties  are 
known.  The  alkali  and  some  of  the  silica  are  removed, 
and  the  mineral  passes  into  clay  or  kaolin  (j).  140). 

Sanidine,  the  clear  glassy  fissured  variety  of  ortho- 
clase  so  conspicuous  in  the  more  silicated  Tertiary  and 
modern  lavas,  occurs  in  some  trachytes  in  large  flat  tables 
(hence  the  name  "sanidine");  more  commonly  in  fine  clear 
or  gray  crystals  or  crystalline  granules;  an  eminently  vol- 
canic mineral. 

Plagioclase  (Triclinic)  Felspars. — While  the  different  felspars 
which  crystallize  in  the  triclinic  system  may  be  more  or  less 

48  On  microscopic  determination  of  felspars,  see  Fouque  and  Michel-Levj, 
op.  cit.  pp.  209,  227,  and  postea,  pp.  108-172. 


132  TEXT-BOOK    OF   GEOLOGY 

easily  distinguished  in  large  crystals  or  crystalline  aggre- 
gates, they  are  difficult  to  separate  in  the  minute  forms  in 
which  they  commonly  occur  as  rock  constituents.  They 
have  been  grouped  oy  petrographers  under  the  general 
name  Plagioclase  (with  oblique  cleavage),  proposed  by 
Tschermak,  who  regards  them  as  mixtures  in  various  pro- 
portions of  two  fundamental  compounds — albite  or  soda- 
felspar,  and  anorthite  or  lime-felspar. 

They  occur  mostly  in  well-developed  crystals,  partly  in 
irregular  crystalline  grains,  crystallites  or  microlites.  On 
a  fresh  fracture,  their  crystals  often  appear  as  clear  glassy 
strips,  on  which  may  usually  be  detected  a  fine  parallel 
lineation  or  ruling,  indicating  a  characteristic  polysynthetio 
twinning  which  never  appears  in  orthoclase.  A  felspar 
striated  in  this  manner  can  thus  be  at  once  pronounced  to 
be  a  triclinic  form,  though  the  distinction  is  not  invariably 
present.  Under  the  microscope,  the  fine  parallel  lamella- 
tion  or  striping,  best  seen  with  polarized  light,  forms  one 
of  the  most  distinctive  features  of  this  group  of  felspars. 
The  chief  triclinio  felspars  are,  Microcline  (potash -felspar, 
KjAlfSi6Olg),  which  occurs  in  granites,  particularly  as  the 
common  felspar  of  the  graphic  varieties;  also  in  some 
gneisses,  etc.;  Albite  (soda-felspar,  Na2O  11-82,  A18O8 
18-56,  SisO  68-62),  found  in  some  granites,  and  in  several 
volcanic  rocks;  Oligoclase  (soda-lime  and  lime-soda  felspars, 
Na20  8-2,  CaO  4-8,  AL,O3  23-0,  SiO,  62-8)  occurs  in  many 

?ranites  and  other  eruptive  rocks;  Andesine  (NasO  7"7,  CaO 
•0,  Al2O825-6,  SiO,  60-0),  observed  in  some  syenites,  etc.; 
Labradorite  (Na,O  4-6,  GaO  12-4,  A18F8  30-2,  SiO252-9),  an 
essential  constituent  of  many  lavas,  etc.,  abundant  in  masses 
in  the  azoic  rocks  of  Canada,  etc. ;  Anorthite  (lime-felspar, 
CaO  20-10,  A1A  36-82,  SiO2  43-08)  found  in  many  volcanic 
rocks,  sometimes  in  granites  and  metamorphic  rocks. 

The  triclinic  felspars  have  been  produced  sometimes 
directly  from  igneous  fusion,  as  can  be  studied  in  many 
lavas,  where  often  one  of  the  first  minerals  to  appear  in 
the  devitrification  of  the  original  molten  glass  has  been  the 
labradorite  or  other  plagioclase.  In  other  cases,  they  have 
resulted  from  the  operation  of  the  processes  to  which  the 
formation  of  the  crystalline  schists  was  due;  large  beds  as 
well  as  abundant  diffused  strings,  veinings,  and  crystals  of 
triclinic  felspar  (labradorite)  form  a  marked  feature  among 
the  ancient  gneisses  of  Eastern  Canada.  The  more  highly 
silicated  species  (albite,  oligoclase)  occur  with  orthoclase  as 


GEOGNOSY  133 

essential  constituents  of  many  granites  and  other  plutonic 
rocks.  The  more  basic  forms  (labradorite,  anorthite)  are  gen- 
erally absent  where  free  silica  is  present;  bat  occur  in  the 
more  basic  igneous  rocks  (basalts,  etc.). 

Considerable  differences  are  presented  by  the  triclinic 
felspars  in  regard  to  weathering.  On  an  exposed  face  of 
rock  they  lose  their  glassy  lustre  and  become  white  and 
opaque.  This  change,  as  in  orthoclase,  arises  from  loss  of 
bases  and  silica,  and  from  hydration.  Traces  of  carbonates 
may  often  be  observed  in  weathered  crystals.  The  original 
steam  cavities  of  old  volcanic  rocks  have  generally  been 
filled  with  infiltrated  minerals,  which  in  many  cases  have 
resulted  from  tbe  weathering  and  decomposition  of  the  tri- 
clinic felspars.  Calcite,  prennite,  and  the  family  of  zeolites 
have  been  abundantly  produced  in  this  way.  The  student 
will  usually  observe  that  where  these  minerals  abound  in 
the  cells  and  crevices  of  a  rock,  the  rock  itself  is  for  the 
most  part  proportionately  decomposed,  showing  the  relation 
that  subsists  between  infiltration-products  and  the  decom- 
position of  the  surrounding  mass.  Abundance  of  calcite  in 
veins  and  cavities  of  a  felspathic  rock  affords  good  ground 
for  suspecting  the  presence  in  the  latter  of  a  lime-felspar." 
(See  under  "  Albitization, "  postea,  p.  1040.) 

Saussurite,  formerly  described  as  a  distinct  mineral 
species,  is  now  found  to  be  the  result  of  the -decomposi- 
tion of  felspars,  which  have  thus  acquired  a  dull  white 
aspect  and  contain  secondary  crystallizations  (zoisite)  out 
of  the  decomposed  substance  of  the  original  felspar.  Such 
saussuritic  felspars  occur  in  varieties  of  gabbro  and  diorite. 
Under  the  microscope  they  present  a  confused  aggregate  of 
crystalline  needles  and  granules  imbedded  in  an  amorphous 
matrix.  (See  postea,  p.  1040.) 

Lcucite  (KSO  21-58,  A15O3  28-50,  SiO2  54-97)  is  a  markedly 
volcanic  mineral,  occurring  as  an  abundant  constituent  of 
many  ancient  and  modern  Italian  lavas,  and  in  some  vari- 
eties of  basalt.  Under  the  microscope,  sections  of  this  min- 
eral are  eight-sided  or  nearly  circular,  and  very  commonly 
contain  inclosures  of  magnetite,  etc.,  conforming  in  arrange- 
ment to  the  external  form  of  the  crystal  or  disposed  radially. 

Nepheline  (Na,O  17-04,  A1.O,  85-26,  KSO  6-46,  SiO,  41-24), 
essentially  a  volcanic  mineral,  being  an  abundant  constit- 

88  A  valuable  essay  on  the  stages  of  the  weathering  of  triclinic  felspar  as 
revealed  by  the  microscope  was  published  by  Q-.  Rose  in  1867.  Zeitsck, 
Deutech.  GeoL  Ges.  xix.  p.  276. 


134  TEXT-BOOK   OF   GEOLOGY 

uent  of  phonolite,  of  some  Vesuvian  lavas,  and  of  some 
forms  of  basalt,  presents  under  the  microscope  various  six- 
sided  and  even  four-sided  forms,  according  to  the  angles 
at  which  the  prisms  are  cut.84  Under  the  name  of  JElceolite 
are  comprised  the  greenish  or  reddish,  dull,  greasy-lustred, 
compact  or  massive  varieties  of  nepheline,  which  occur  in 
some  syenites  and  other  ancient  crystalline  rocks. 

THE  MICA  FAMILY  embraces  a  number  of  minerals, 
distinguished  especially  by  their  very  perfect  basal  cleav- 
age, whereby  they  can  be  split  into  remarkably  thin  elastic 
laminae,  and  by  a  predominant  splendent  pearly  lustre. 
They  consist  essentially  of  silicates  of  alumina,  magnesia, 
iron  and  alkalies,  and  may  be  conveniently  divided  into 
two  groups,  the  white  micas,  which  are  silicates  of  alumina 
with  alkalies,  iron  and  magnesia,  and  the  black  micas,  in 
which  the  magnesia  and  iron  play  a  more  conspicuous  part. 

Muscovite  (Potash-mica,  white  mica,  Glimmer,  K2O  3'07- 
12-44,  Na20  0-4-10,  FeO  0-1-16,  Fe2O3  0-46-8-80,  MgO 
0-37-3-08,  A1A  28-05-38-41,  SiO,  43-47-51-73,  H2O  0-98- 
6-22),  abundant  as  an  original  constituent  of  many  crystal- 
line rocks  (granite,  etc.),  and  as  one  of  the  characteristic 
minerals  of  the  crystalline  schists;  also  in  many  sandstones, 
where  its  small  parallel  flakes,  derived,  like  the  surround- 
ing quartz  grains,  from  older  crystalline  masses,  impart  a 
silvery  or  "micaceous"  lustre  and  fissility  to  the  stone.** 
The  persistence  of  muscovite  under  exposure  to  weather  is 
shown  by  the  silvery  plates  of  the  mineral,  which  may  be 
detected  on  a  crumbling  surface  of  granite  or  schist  where 
most  of  the  other  minerals,  save  the  quartz,  have  decayed; 
also  by  the  frequency  of  the  micaceous  lamination  of 
sandstones. 

Biotite  (Magnesia-mica,  black  mica,  MgO  10-30  per  cent) 
occurs  abundantly  as  an  original  constituent  of  many 
granites,  gneisses,  and  schists;  also  sometimes  in  basalt, 
trachyte,  and  as  ejected  fragments  and  crystals  in  tuff.  Its 
small  scales,  when  cut  transverse  to  the  dominant  cleavage, 
may  usually  be  detected  under  the  microscope  by  their  re- 
markably strong  dichroism,  their  fine  parallel  lines  of  cleav- 
age, and  their  frequently  frayed  appearance  at  the  ends. 
Under  the  action  of  the  weather  it  assumes  a  pale,  dull,  soft 

24  On  the  microscopic  distinction  between  nepheline  and  apatite,  see  Fouque" 
and  Michel-LeVy,  "Mineral.  Micrograph."  p.  276. 

25  On  the  microscopic  determination  of  the  micas,  see  Fouque  and  Michel- 
Levy,  op.  cit.  p.  333. 


GEOGNOSY  135 

crust,  owing  to  removal  of  its  bases.  The  mineral  rubellan, 
which  occurs  in  hexagonal  brown  or  red  opaque  inelastic 
tables  in  some  basalts  and  other  igneous  rooks,  is  regarded 
as  an  altered  form  of  biotite. 

Phlogopite  is  another  dark  ferro-magnesian  mica  which 
contains  a  little  fluorine.  Lepidolite  (Lithia-mica)  occurs  in 
some  granites  and  crystalline  schists,  especially  in  veins. 
Damourite,  merely  a  variety  of  muscovite,  occurs  among 
crystalline  schists.  Sericite,  a  talc-like  variety  of  musco- 
vite, occurs  in  soft  inelastic  scales  in  many  schists,  as  a 
result  of  the  alteration  of  orthoclase  felspar.*'  Margarodite, 
a  silvery  talc-like  hydrous  mica,  is  widely  diffused  as  a 
constituent  of  granite  and  other  crystalline  rocks.  Para- 
gonite,  a  scaly  micaceous  mineral,  forms  the  main  mass  of 
certain  alpine  schists. 

Hornblende  (Monoclinic  Amphibole,  CaO,  10-12,  MgO  11- 
24,  FeAO-10,  A18O3  5-18,  SiO4  40-^50  also  usually  with 
some  Na8O,  KS0  and  FeO).  Divided  into  two  groups.  1st. 
Non-aluminous,  including  the  white  and  pale  green  or  gray 
fibrous  varieties  (tremolite,  actinolite,  etc.).  2d.  Aluminous, 
embracing  the  more  abundant  dark  green,  brown,  or  black 
varieties.  Under  the  microscope,  hornblende  presents 
cleavage-angles  of  124°  30',  the  definite  cleavage-planes 
intersecting  each  other  in  a  well-marked  lattice  work, 
sometimes  with  a  finely  fibrous  character  superadded.  It 
also  shows  a  marked  pleochroism  with  polarized  light, 
which,  as  Tschermak  first  pointed  out,  usually  distin- 
guishes it  from  augite."  Hornblende  has  abundantly  re- 
sulted from  the  alteration  (paramorphism)  of  augite  (see 
below,  Uralite).  In  many  rocks  the  ferro-magnesian  sili- 
cate which  is  now  hornblende  was  originally  augite;  the 
epidiorites,  for  instance,  were  probably  once  dolerites  or 
allied  pyroxenic  rocks.  The  pale  non-aluminous  horn- 
blendes are  found  among  gneisses,  crystalline  limestones, 
and  other  metamorphic  rocks.  The  dark  varieties,  though 
also  found  in  similar  situations,  sometimes  even  forming 
entire  masses  of  rock  (amphibolite,  hornblende-rock,  horn- 
blende-schist), are  the  common  forms  in  granitic  and  vol- 
canic rocks  (syenite,  diorite,  hornblende-andesite,  etc.).  The 
former  group  naturally  gives  rise  by  weathering  to  various 


36  On  the  occurrence  of  this  mineral  in  schists,  see  Lessen,  Zeitsch.  Deutsch. 
Geol.  Ges.  1867,  pp.  546,  661. 

*7  Wien.  Acad.  May,  1869.  See  also  Fouque  and  Michel-LeVy,  op.  cit.  pp. 
349,  365. 


136  TEXT- BOOK   OF  QEOLOQY 

hydrous  magnesian  silicates,  notably  to  serpentine  and  talc. 
In  the  weathering  of  the  aluminous  varieties,  silica,  lime, 
magnesia,  and  a  portion  of  the  alkalies  are  removed,  with 
conversion  of  part  of  the  earths  and  the  iron  into  carbon- 
ates. The  further  oxidation  of  the  ferrous  carbonate  is 
shown  by  the  yellow  and  brown  crust  so  commonly  to  be 
seen  on  the  surface  or  penetrating  cracks  in  the  hornblende. 
The  change  proceeds  until  a  mere  internal  kernel  of  unal- 
tered mineral  remains,  or  until  the  whole  has  been  converted 
into  a  ferruginous  clay. 

Anthophyflite  (Khombic  Amphibple  (MgFe)SiO8)  is  a  min- 
eral which  occurs  in  bladed,  sometimes  rather  fibrous  forms, 
among  the  more  basic  parts  of  old  gneisses;  also  in  zones 
of  alteration  round  some  of  the  ferro-magnesian  minerals  of 
certain  gabbros. 

Soda-amphiboles  resemble  ordinary  hornblende,  but,  as  their 
name  denotes,  they  contain  a  more  marked  proportion  of 
soda.  They  include  a  blue  variety  called  Glaucophane, 
which  is  found  abundantly  in  certain  schists;  Riebeckite, 
which  is  also  blue  and  occurs  in  some  granites  and  micro- 
granites;  Arfvedsonite,  a  dark  greenish  or  brown  variety. 

Uralite  is  the  name  given  to  a  mineral  which  was  origi- 
nally pyroxene,  but  has  now  by  a  process  of  paramorphism 
acquired  the  internal  cleavage 'and  structure  of  hornblende 
(amphibole).  Under  the  microscope  a  still  unchanged  ker- 
nel of  pyroxene  may  in  some  specimens  be  observed  in 
the  centre  of  a  crystal  surrounded  by  strongly  pleochroic 
hornblende,  with  its  characteristic  cleavage  and  actinolitic 
needles  (postea,  p.  1040).  Smaragdite  is  a  beautiful  grass- 
green  variety  also  resulting  from  the  alteration  of  a  pyroxene. 

Augite  (Monoclinic  Pyroxene,  CaO  12-27-5,  MgO  3-22 -5, 
FeO  1-34,  FeA  0-10,  ALA  0-11;  SiO,  40-57-4).  Divided 
like -hornblende  into  two  groups.  1st.  Non-aluminous,  with 
a  prevalent  green  color  (malacolite,  coccolite,  diopside,  sah- 
lite,  etc.).  2d.  Aluminous,  including  generally  the  dark 
green  or  black  varieties  (common  augite,  fassaite).  It  would 
appear  that  the  substance  of  hornblende  and  augite  is 
dimorphous,  for  the  experiments  of  Berthier,  Mitscherlich 
and  Q-.  Kose  showed  that  hornblende,  when  melted  and 
allowed  to  cool,  assumed  the  crystalline  form  of  augite; 
whence  it  has  been  inferred  that  hornblende  is  the  result 
of  slow,  and  augite  of  comparatively  rapid  cooling.118  Under 

28  The  same  results  have  been  obtained  recently  by  Fouque  and  Michel-Levy, 
"Synthese  des  Mine'raux  et  des  Roches,"  1882,  p.  78. 


GEOGNOSY  137 

the  microscope,  augite  in  thin  slices  is  only  very  feebly 
pleocbroic,  and  presents  cleavage  lines  intersecting  at  an 
angle  of  87°  5'.  It  is  often  remarkable  for  the  amount  of  ex- 
traneous materials  inclosed  within  its  crystals.  Like  some 
felspars,  augite  may  be  found  in  basalt  with  merely  an  outer 
casing  of  its  own  substance,  the  core  being  composed  of 
magnetite,  of  the  ground-mass  of  the  surrounding  rock, 
or  of  some  other  mineral  (Fig.  7).  The  distribution  of  au- 
gite resembles  that  of  hornblende;  the  pale,  non-aluminous 
varieties  are  more  specially  found  among  gneisses,  marbles, 
and  other  crystalline,  foliated,  or  metamorphic  rocks;  the 
dark-green  or  black  varieties  enter  as  essential  constituents 
into  many  igneous  rocks  of  all  ages,  from  Paleozoic  up  to 
recent  times  (diabase,  basalt,  andesite,  etc.).  Its  weather- 
ing also  agrees  with  that  of  hornblende.  The  aluminous 
varieties,  containing  usually  some  lime,  give  rise  to  calca- 
reous and  ferruginous  carbonates,  from  which  the  fine  inter- 
stices and  cavities  of  the  surrounding  rock  are  eventually 
filled  with  threads  and  kernels  of  calcite  and  strings  of 
hydrous  ferric  oxide.  In  basalt  and  dolerite,  for  example, 
the  weathered  surface  often  acquires  a  rich  yellow  color 
from  the  oxidation  and  hyd ration  of  the  ferrous  oxide. 

Omphacite,  a  granular  variety  of  pyroxene,  grass  green 
in  color,  and  commonly  associated  with  red  garnet  in  the 
rock  known  as  eclogite. 

Diallage,  a  variety  of  augite,  characterized  by  its  some- 
what metallic  lustre  and  foliated  aspect,  is  especially  a  con- 
stituent of  gabbro. 

Rhombic-Pyroxenes.  —  There  are  three  rhombic  forms  of 
pyroxene,  which  occur  as  important  constituents  of  some 
rocks,  Enstatite,  Bronzite  and  Hypersthene.  Enstatite  oc- 
curs in  Iherzolite,  serpentine,  and  other  olivine  rocks;  also 
in  meteorites.  Bronzite  is  found  under  similar  conditions 
to  enstatite,  from  which  it  is  with  difficulty  separable.  It 
occurs  in  some  basalts  and  in  serpentines;  also  in  meteor- 
ites. Bronzite  and  enstatite  weather  into  dull  green  ser- 
pentinous  products.  Bastite  or  Schiller-spar  is  a  frequent 
product  of  the  alteration  of  Bronzite  or  Enstatite,  and  may 
be  observed  with  its  characteristic  pearly  lustre  in  serpen- 
tine. Hypersthene  occurs  in  hypersthenite  and  hypersthene- 
andesite;  also  associated  with  other  magnesian  minerals 
among  the  crystalline  schists. 

Olivine  (Peridot,  MgO  32-4-50-5,  FeO  6-29-7,  SiO.81-6- 
42-8)  forms  an  essential  ingredient  of  basalt,  likewise  the 
main  part  of  various  so-called  olivine-rocks  or  peridotitea 


138  TEXT-BOOK   OF   GEOLOGY 

(as  Iherzolite  and  pikrite),  and  occurs  in  many  gabbros; 
under  the  microscope  with  polarized  light,  gives,  when 
fresh,  bright  colors,  specially  red  and  green,  but  it  is  not 
perceptibly  pleochroic.  Its  orthorhombic  outlines  can  some- 
times be  readily  observed,  but  it  often  occurs  in  irregularly 
shaped  granules  or  in  broken  crystals,  and  is  liable  to  be 
traversed  by  fine  fissures,  which  are  particularly  developed 
transverse  to  the  vertical  axis.  It  is  remarkably  prone  to 
alteration.  The  change  begins  on  the  outer  surface  and 
extends  inward  and  specially  along  the  fissures,  until  the 
whole  is  converted  either  into  a  green  granular  or  fibrous 
substance,  which  is  probably  in  most  cases  serpentine  (Fig. 
26),  or  into  a  reddish-yellow  amorphous  mass  (limonite). 

Hauyne  (SiO8  34-06,  Al  27'64,  Na?O  11-79,  K2O  4-96,  CaO 
10-60,  SO4 11-25)  occurs  abundantly  in  Italian  lavas,  in  basalt 
of  the  Bifel,  and  elsewhere. 

Nosean  (SiO,  83-79,  Al  28-76,  Na2O  26-20,  SO4  11-26), 
under  the  microscope,  is  one  of  the  most  readily  recognized 
minerals,  showing  a  hexagonal  or  quadrangular  figure,  with 
a  characteristic  broad  dark  border  corresponding  to  the  ex- 
ternal contour  of  the  crystal,  and  where  weathering  has  not 
proceeded  too  far,  inclosing  a  clear  colorless  centre.  It  oc- 
curs in  minute  forms  in  most  phonolites,  also  in  large  crys- 
tals in  some  sanidine  volcanic  rocks.  Both  hauyne  and 
nosean  are  volcanic  minerals  associated  with  the  lavas  of 
more  recent  geological  periods. 

Epidote  (Pistacite,  CaO  16-30,  MgO  0-4-9,  Fe2O3  7-5-17-24, 
A12O8  14-47-28-9,  SiO2  33-81-57-65)  occurs  in  many  crystal- 
line rocks,  as  a  result  of  the  alteration  of  other  silicates  such 
as  felspars  and  hornblende  (see  postea,  p.  1040);  largely  dis- 
tributed in  certain  schists  and  quartzites,  sometimes  associ- 
ated with  beds  of  magnetite  and  hematite. 

Zoisite  is  allied  to  epidote  but  contains  no  iron.  It  occurs 
in  altered  basic  igneous  rocks  and  also  (sometimes  in  large 
aggregations)  in  metamorphic  groups. 

Vesuvianite  (Idocrase,  CaO  27-7-37-6,  MgO  0-10-6,  FeO 
0-16,  A12O8  10-5-26-1,  SiO2  35-39-7,  H2O  0-2-73)  occurs  in 
ejected  blocks  of  altered  limestone  at  Somma,  also  among 
crystalline  limestones  and  schists. 

'  Andalusite  (A19O3  50-96-62-2,  Fe2O3  0-5-7,  SiO8  35-3-40-17). 
—  Found  in  crystalline  schists.  The  variety  Chiastolite, 
abundant  in  some  dark  clay-slates,  is  distinguished  by  the 
regular  manner  in  which  the  dark  substance  of  the  surround- 
ing matrix  has  been  inclosed,  giving  a  cross-like  transverse 
section.  These  crystals  have  been  developed  in  the  rock 


GEOGNOSY  139 

after  its  formation,  and  are  regarded  as  proofs  of  contact- 
metamorphism.  (Book  IV.  Part  VIII.)  Sillimanite  or  Fi~ 
brolite  is  the  name  given  to  a  fibrous  variety  which  is  not 
infrequent  among  schistose  rocks. 

Dichroite  (Cordierite,  lolite,  MgO  8-2-20-45,  FeO  0-11-58, 
A12O,  28-72-33-11,  Si02  48-1-50-4,  H80  0-2-66)  occurs  in 
gneiss,  sometimes  in  large  amount  (cordierite-gneiss);  occa- 
sionally as  an  accessory  ingredient  in  some  granites;  also  in 
talc-schist.  Undergoes  numerous  alterations,  having  been 
found  changed  into  pinite,  chlorophyllite,  mica,  etc. 

Scapolites,  a  series  of  minerals  consisting  of  silicates  of 
alumina,  lime  and  soda,  with  a  little  chlorine.  They  are 
found  among  the  cavities  of  lavas,  but  more  frequently 
among  metamorphic  rocks,  where  they  appear  in  associa- 
tion with  altered  felspars.  Dipyre,  Couseranite  and  Meionite 
are  varieties  of  the  series. 

Kyanite  (Al2SiO5)  occurs  in  bladed  aggregates  of  a  beauti- 
ful delicate  blue  color  among  schistose  rocks;  also  in  granu- 
lar forms. 

Garnet  (CaO  0-5-78,  MgO  0-10-2,  Fe2O3  0-6-7,  FeO  24-82- 
89-68,  MnO  0-6-43,  A18O3  15-2-21-49,  SiO2  35-75-52-11.— 
The  common  red  and  brown  varieties  occur  as  essential  con- 
stituents of  eclogite,  garnet- rock;  and  often  as  abundant 
accessories  in  mica-schist,  gneiss,  granite,  etc.  Under  the 
microscope,  garnet  as  a  constituent  of  rocks,  presents  three- 
sided,  four-sided,  six-sided,  eight-sided  (or  even  rounded) 
figures  according  to  the  angle  at  which  the  individual  crys- 
tals are  cut;  it  is  usually  clear,  but  full  of  flaws  or  of  cavi- 
ties; passive  in  polarized  light. 

Tourmaline  (Schorl,  CaO  0-2-2,  MgO  0-14-89,  Na2O  0-4-95, 
K2O  0-3-59,  FeO  0-12,  Fe2O3  0-13-08,  A12O3  30-44-44-4,  SiO, 
35-2-41-16,  B  3-63-11-78,  F  1-49-2-58),  with  quartz,  forms 
tourmaline-rock;  associated  with  some  granites;  occurs  also 
diffused  through  many  gneisses,  schists,  crystalline  lime- 
stones, and  dolomites,  likewise  in  sands  (see  Zircon).  Pleo- 
chroism  strongly  marked. 

Zircon  (ZrO,  63-5-67-16,  Fe2O3  0-2,  SiO2  32-35-26)  occurs 
as  a  chief  ingredient  in  the  zircon-syenite  of  Southern  Nor- 
way; frequent  in  granites,  diorites,  gneisses,  crystalline  lime- 
stones and  schists;  in  eclogite;  as  clear  red  grains  in  some 
basalts,  and  also  in  ejected  volcanic  blocks;  of  common  oc- 
currence in  sands,  clays,  sandstones,  shales  and  other  sedi- 
mentary rocks  derived  from  crystalline  masses  such  as 
granite,  etc. 

Thamte  (Sphene,  CaO  2176-33,  TiO2  33-43-5,  SiO2  30-35), 


140  TEXT-BOOK   OF   GEOLOGY 

dispersed  in  small  characteristically  lozenge-shaped  crystals 
in  many  syenites,  also  in  granite,  gneiss,  and  in  some  vol- 
canic rocks  (basalt,  trachyte,  phpnoiite). 

Zeolites.— Under  this  name  is  included  a  characteristic 
family  of  minerals,  which  have  resulted  from  the  alteration, 
and  particularly  from  the  hydra tion,  of  other  minerals,  es- 
pecially of  felspars.  Secondary  products,  rather  than  origi- 
nal constituents  of  rocks,  they  often  occur  in  cavities  both 
as  prominent  amygdales  and  veins,  and  in  minute  interstices 
only  perceptible  by  the  microscope.  In  these  minute  forms 
they  very  commonly  present  a  finely  fibrous  divergent  struc- 
ture. As  already  remarked,  a  relation  may  often  be  traced 
between  the  containing  rock  and  its  inclosed  zeolites.  Thus 
among  the  basalts  of  the  Inner  Hebrides,  the  dirty  green 
decomposed  amygdaloidal  sheets  are  the  chief  repositories 
of  zeolites,  while  the  firm,  compact,  columnar  beds  are  com- 
paratively free  from  these  alteration  products."  Among  the 
more  common  zeolites  are  Analcime,  Natrolite,  Prehnite  and 
Stilbite. 

Kaolin  (A1203  38-6-40-7,  CaO  0-3-5,  K2O  0-1-9,  SiO,45'5- 
46-53,  H2O  9-14-54)  results  from  the  alteration  of  potash-  and 
soda-felspars  exposed  to  atmospheric  influences.  Under  the 
microscope  the  fine  white  powdery  substance  is  found  to  in- 
clude abundant  minute  six-sided  colorless  plates  and  scales 
which  have  been  formed  by  recrystallization  of  the  decom- 
posed substance  of  the  felspar.  The  purest  white  kaolin  is 
called  china-clay,  from  its  extensive  use  in  the  manufacture 
of  porcelain.  Ordinary  clay  is  impure  from  admixture  of 
iron,  lime*,  and  other  ingredients,  among  which  the  debris  of 
the  undecomposed  constituents  of  the  original  rock  may 
form  a  marked  proportion. 

Talc  (MgO  23-19-35-4,  FeO  0-4-5,  A12O3  0-5-67,  SiO, 
56-62-64-53,  H2O  0-6-65)  occurs  as  an  essential  constitu- 
ent of  talc-schist,  and  as  an  alteration  product  replacing 
mica,  hornblende,  augite,  olivine,  diallage,  and  other  min- 
erals in  crystalline  rocks. 

Chlorite  (MgO  24-9-36,  FeO  0-5-9,  FeA -0-11-84  A1203 
10-5-19-9,  SiO2  30-33-5,  H8O  11'5-16)>  including  several 
varieties  or  species,  occurs  in  small  green  hexagonal  tables 
or  scaly  vermicular  or  earthy  aggregates ;  is  an  essential  in- 
gredient of  chlorite-schist,  and  occurs  abundantly  as  an 
alteration  product  (of  hornblende,  etc.)  in  fine  filaments, 
incrustations,  and  layers  in  many  crystalline  rocks.  (See 

89  See  Sullivan  in  Jukes'  "Manual  of  Geology,"  p.  85. 


GEOGNOSY  Ul 

under  "Chloritization,"  postea,  p.  1040.)  Among  the  min- 
erals grouped  under  the  general  head  of  chlorites  are  Ghloro- 
phceite,  Clinochlore,  Delessite,  Pennine,  Ripidolite,  and  others. 

Ottrelite  (Chloritoid,  H20  (FeMg)  AlaSiO7)  occurs  in  small 
lustrous  iron-black  or  greenish-black  lozenge-shaped  or  six- 
side  plates  in  certain  schists.  It  resembles  chlorite  but  is  at 
once  distinguishable  from  that  mineral  by  its  much  greater 
hardness. 

Serpentine  (MgO  28^,  FeO  1-10-8,  A1,O3  0-6-5,  SiO,  37-5- 
44-5,  H2O  9-5-14*6)  is  a  product  of  the  alteration  of  pre-ex- 
isting minerals,  and  especially  of  olivine.  It  occurs  in  nests, 
grains,  threads,  and  veins  in  rocks  which  once  contained 
olivine30  (p.  138),  also  massive  as  a  rock,  in  which  it  has  re- 
placed olivine,  enstatite  or  some  other  magnesian  bisilicate 
(pp.  300,  1040).  Under  the  microscope  it  presents,  in  very 
thin  slices,  a  pale  leek-green  or  bluish-green  base,  showing 
aggregate  polarization.  Through  this  base  runs  a  network 
of  dark  opaque  threads  and  veinings.  Sometimes  among 
these  veinings,  or  through  the  network  of  green  serpentinous 
matter  in  the  base,  the  forms  of  original  olivine  crystals  may 
be  traced  (Figs.  26,  27). 

Glaucomte(CaO  0-4-9,  MgO  0-5-9,  K  O  0-12-9,  Na,O  0-2-5, 
FeO  3-25-5,  FeaO3  0-28-1,  A12O3  1-5-13-3,  SiO,  46-5-60-09, 
H2O  0-14-7).  Found  in  many  stratified  formations,  particu- 
larly among  sandstones  and  limestones,  where  it  envelops 
grains  of  sand,  or  fills  and  coats  foraminifera  and  other  or- 
ganisms, giving  a  general  green  tint  to  the  rock.  It  is  at 
present  being  formed  on  the  sea-floor  off  the  coasts  of  Geor- 
gia and  South  Carolina,  where  Pourtales  found  it  filling  the 
chambers  of  recent  polythalamia. 

6.  CARBONATES.  This  familv  of  minerals  furnishes  only 
four  which  enter  largely  into  the  formation  of  rocks,  viz. 
Carbon-ate  of  Calcium  in  its  two  forms,  Calcite  and  Arago- 
nite,  Carbonate  of  Magnesium  (and  Calcium)  in  Dolomite, 
and  Carbonate  of  Iron  in  Siderite. 

Calcite  (CaCOj)  occurs  as  (1)  an  original  constituent  of 
many  aqueous  rocks  (limestone,  calcareous  shale,  etc.), 
either  as  a  result  of  chemical  deposition  from  water  (calc- 
sinter,  stalactites,  etc.),  or  as  a  secretion  by  plants  or  ani- 
mals;81 or  (2)  as  a  secondary  product  resulting  from  weath- 


30  See  Tschermak,  Wien.  Akad.  Ivi.  1867. 

81  Mr.  Sorby  has  investigated  the  condition  in  which  the  calcareous  matter 
of  the  harder  parts  of  invertebrates  exists.  He  finds,  that  in  forarainifera, 
echinoderms,  brachiopods,  Crustacea,  and  some  lamellibranchs  and  gasteropoda, 


142  TEXT-BOOK   OF   GEOLOGY 


ering,  when  it  is  found  filling  or  lining  cavities,  or  diffused 
through  the  capillary  interstices  of  minerals  and  rocks.  It 
probably  never  occurs  as  an  original  ingredient  in  the  mas- 
sive crystalline  rocks,  such  as  granite,  felsite,  and  lavas. 
Under  the  microscope,  calcite  is  readily  distinguishable  by 
its  intersecting  cleavage  lines,  by  a  frequent  twin  lamella- 
tion  (sometimes  giving  interference  colors),  strong  double 
refraction,  weak  or  inappreciable  pleochroism,  and  charac- 
teristic iridescent  polarization  tints  of  gray,  rose  and  blue. 

From  the  readiness  with  which  water  absorbs  carbon- 
dioxide,  from  the  increased  solvent  power  which  it  thereby 
acquires,  and  from  the  abundance  of  calcium  in  various 
forms  among  minerals  and  rocks,  it  is  natural  that  calcite 
should  occur  abundantly  as  a  pseudomorph  replacing  other 
minerals.  Thus,  it  has  been  observed  taking  tne  place  of  a 
number  of  silicates,  as  orthoclase,  oligoclase,  garnet,  augite 
and  several  zeolites;  of  the  sulphates,  anhydrite,  gypsum, 
barytes,  and  celestine;  of  the  carbonates,  aragonite,  dolo- 
mite, cerussite;  of  the  fluoride,  fluor-spar;  and  of  the  sul- 
phide, galena.  Moreover,  in  many  massive  crystalline  rocks 
(diorite,  dolerite,  etc.),  which  have  been  long  exposed  to  at- 
mospheric influence,  this  mineral  may  be  recognized  by  the 
brisk  effervescence  produced  by  a  drop  of  acid,  and  in 
microscopic  sections  it  appears  filling  the  crevices,  or  send- 
ing minute  veins  among  the  decayed  mineral  constituents. 
Calcite  is  likewise  the  great  petrifying  medium:  the  vast 
majority  of  the  animal  remains  found  in  the  rocky  crust  of 
the  globe  have  been  replaced  by  calcite,  sometimes  with  a 
complete  preservation  of  internal  organic  structure,  some- 
times with  a  total  substitution  of  crystalline  material  for 
that  structure,  the  mere  outer  form  of  the  organism  alone 
surviving." 

Aragonite  (CaCO8),  harder,  heavier,  and  much  less  abun- 
dant than  calcite,  which  is  the  more  stable  form  of  calcium- 
carbonate;  occurs  with  beds  of  gypsum,  also  in  mineral 
veins,  in  strings  running  through  basalt  and  other  igneous 
rocks,  and  in  the  shells  of  many  mollusca.  It  is  thus  always 
a  deposit  from  water,  sometimes  from  warm  mineral  springs, 
sometimes  as  the  result  of  the  internal  alteration  of  rocks, 


it  occurs  as  calcite;  tbat  in  nautilus,  sepia,  most  gasteropoda,  many  lamelli- 
branchs,  etc.,  it  is  aragonite;  and  that  in  not  a  few  cases  the  two  forms  occur 
together,  or  that  the  carbonate  of  lime  is  hardened  by  an  admixture  of  phos- 
phate.    Quart.  Journ.  Geol.  Soc.  1879.     Address,  p.  61. 
32  See  index  sub  voc.  Calcite. 


GEOGNOSY  143 

and  sometimes  through  the  action  of  living  organisms.  Be- 
ing more  easily  soluble  than  calcite,  it  has  no  doubt  in  many 
cases  disappeared  from  limestones  originally  formed  mainly 
of  aragonite  shells,  and  has  been  replaced  by  the  more  dur- 
able calcite,  with  a  consequent  destruction  of  the  traces  of 
organic  origin.  Hence  what  are  now  thoroughly  crystalline 
limestones  may  have  been  formed  by  a  slow  alteration  of 
such  shelly  deposits  (p.  811). 

Dolomite  (Bitter-spar  (Ca;  Mg)CO3,  p.  264)  occurs  (1)  as 
an  original  deposit  in  massive  beds  (magnesian  limestone), 
belonging  to  many  different  geological  formations;  (2)  as  a 
product  of  alteration,  especially  of  ordinary  limestone  or  of 
aragonite  (Dolomitization,  p.  546). 

Siderite  (Brown  Ironstone,  Spathic  Iron,  Chalybite,  Fer- 
rous Carbonate,  FeCO3)  occurs  crystallized  in  association 
with  metallic  ores,  also  in  beds  and  veins  of  many  crystal- 
line rocks,  particularly  with  limestones;  the  compact  argil- 
laceous varieties  (clay-ironstone)  are  found  in  abundant 
nodules  and  beds  in  the  shales  of  Carboniferous  and  other 
formations  where  they  have  been  deposited  from  solution  in 
water  in  presence  of  decaying  organic  matter  (see  pp.  257, 
267). 

7.  SULPHATES.     Among  the  sulphates  of   the   mineral 
kingdom,  only  two  deserve  notice  here  as  important  com- 
pounds in  the  constitution  of  rocks — viz.  calcium-sulphate 
or  sulphate  of  lime  in  its  two  forms,  Anhydrite  and  Gyp- 
sum; and  barium-sulphate  or  sulphate  of  baryta  in  Barytes. 

Anhydrite  (CaSO4)  occurs  more  especially  in  association 
with  beds  of  gypsum  and  rock-salt  (see  p.  265). 

Gypsom  (Selenite,  CaSO4+2HsO).  Abundant  as  an  origi- 
nal aqueous  deposit  in  many  sedimentary  formations  (see 
p.  265). 

Barytes  (Heavy  Spar,  BaSO4).  Frequent  in  veins  and 
especially  associated  with  metallic  ores  as  one  of  their 
characteristic  vein-stones. 

8.  PHOSPHATES.    The  phosphates  which  occur  most  con- 
spicuously as  constituents  or  accessory  ingredients  of  rocks 
are  the  tricalcic  phosphate  or  Apatite,  and  triferrous  phos- 
phate or  Vivianite. 

Apatite  (3Ca,  (PO4)4-CaF,)  occurs  in  many  igneous  rocks 
(granites,  basalts,  etc.),  in  minute  hexagonal  non-pleochroic 
needles,  giving  faint  polarization  tints;  also  in  large  crystals 
and  massive  beds  associated  with  metamorphic  rocks. 

Vivianite  (Blue  iron-earth,  FesPsO8,  8H80)  occurs  crystal- 
lized in  metalliferous  veins;  the  earthy  variety  is  not  infre- 


144  TEXT-BOOK   OF   GEOLOGY 

quent  in  peat-mosses  where  animal  matter  has  decayed,  and 
is  sometimes  to  be  observed  coating  fossil  fishes  as  a  fine 
layer  like  the  bloom  of  a  plum. 

9.  FLUORIDES.      The  element   fluorine,  though   widely 
diffused  in   nature,   occurs  as  an  important  constituent  of 
comparatively  few  minerals.     Its  most  abundant  compound 
is  with  Calcium  as  the  common  mineral  Fluorite.     It  occurs 
also  with  sodium  and  aluminium  in  the  mineral  Cryolite. 

Fluorite  (Fluor-spar,  CaFa)  occurs  generally  in  veins,  espe- 
cially in  association  with  metallic  ores. 

10.  CHLORIDES.     There  is  only  one  chloride  of  impor- 
tance as  a  constituent  of  rocks — sodium-chloride  or  common 
salt  (NaCl),  which,  occurring  chiefly  in  beds,  is  described 
among   the  rocks  at  p.   259.     Carnallite  (KClMgCl,6H.,O), 
a  hydrated  chloride  of  potassium  and  magnesium,  occurs 
in   beds   associated  with   rock-salt,   gypsum,   etc.,  in  some 
salt  districts  (p.  260). 

11.  SULPHIDES.     Sulphur  is  found  united  with  metals 
in   the  form  of   sulphides,    many  of   which  form  common 
minerals.     The  sulphides  of  lead,  silver,  copper,  zinc,  anti- 
mony,   etc.,    are    of    great    commercial   importance.     Iron- 
disulphide,    however,    is   the   only   one   which  merits  con- 
sideration here  as  a  rock-forming  substance.     It  is  formed 
at  the  present  day  by  some  thermal  springs,  and  has  been 
developed  in  many  rocks  as  a  result  of  the  action  of  infil- 
trating water  in  presence  of  decomposing  organic  matter  and 
iron  salts.     It  occurs  in  two  forms,  Pyrite  and  Marcasite. 

Pyrite  (Eisenkies,  Schwefelkies,  FeSa)  occurs  disseminated 
through  almost  all  kinds  of  rocks,  often  in  great  abundance, 
as  among  diabases  and  clay-slates;  also  frequent  in  veins  or 
in  beds.  In  microscopic  sections  of  rocks,  pyrite  appears 
in  small  cubical,  perfectly  opaque  crystals,  which  with  re- 
flected light  show  the  characteristic  brassy  lustre  of  the 
mineral,  and  cannot  thus  be  mistaken  for  the  isometric 
magnetite,  of  which  the  square  sections  exhibit  a  charac- 
teristic blue-black  color.  Pyrite  when  free  from  marcasite 
yields  but  slowly  to  weathering.  Hence  its  cubical  crystals 
may  be  seen  projecting  still  fresh  from  slates  which  have 
been  exposed  to  the  atmosphere  for  several  generations.88 

Marcasite  (Hepatic  pyrites)  occurs  abundantly  among  sedi- 
mentary formations,  sometimes  abundantly  diffused  in  mi- 
nute particles  which  impart  a  blue-gray  tint,  and  speedily 

33  For  an  elaborate  paper  on  the  decomposition  of  Pyrites,  see  A..  A.  Julien, 
Annals  New  York  Acad.  Sci.  vols.  iii.  and  iv. 


GEOGNOSY  145 

weather  yellow  on  exposure  and  oxidation;  sometimes 
segregated  in  layers,  or  replacing  the  substance  of  fossil 
plants  or  animals;  also  in  veins  through  crystalline  rocks. 
This  form  of  the  sulphide  is  especially  characteristic  of 
stratified  fossiliferous  rocks,  and  more  particularly  of  those 
of  Secondary  and  Tertiary  date.  It  is  extremely  liable  to 
decomposition.  Hence  exposure  for  even  a  short  time 
to  the  air  causes  it  to  become  brown;  free  sulphuric  acid 
is  produced,  which  attacks  the  surrounding  minerals,  some- 
times at  once  forming  sulphates,  at  other  times  decomposing 
aluminous  silicates  and  dissolving  them  in  considerable 
quantity.  Dr.  Sullivan  mentions  that  the  water  annually 
pumped  from  one  mine  in  Ireland  carried  up  to  the  surface 
more  than  a  hundred  tons  of  dissolved  silicate  of  alumina.34 
Iron  disulphide  is  thus  an  important  agent  in  effecting  the 
internal  decomposition  of  rocks.  It  also  plays  a  large  part 
as  a  petrifying  medium,  replacing  the  organic  matter  of 
plants  and  animals,  and  leaving  casts  of  their  forms,  often 
with  bright  metallic  lustre.  Such  casts  when  exposed  to 
the  air  decompose. 

Pyrrhotine  (Magnetic  pyrites,  Fe,S8)  is  much  less  abundant 
than  either  of  the  forms  of  ordinary  iron-pyrites,  from  which 
it  is  distinguished  by  its  inferior  hardness  and  its  magnetic 
character. 

It  will  be  observed  that  great  differences  exist  in  the 
relative  importance  of  the  minerals  above  enumerated  as 
constituents  of  rocks.  Prof.  Rosenbusch  points  out  that 
they  may  be  naturally  arranged  in  four  groups — 1st,  ores 
and  accessory  ingredients  (magnetite,  haematite,  ilmenite, 
apatite,  zircon,  spinel,  titanite),  2d,  magnesian  and  ferru- 
ginous silicates  (biotite,  amphibole,  pyroxene,  olivine),  3d, 
felspathic  constituents  (felspar  proper,  nepheline,  leucite, 
melilite,  sodalite,  hauyne),  4th,  free  silica.86 

§  iii.  Determination  of  Rocks 

Rocks  considered  as  mineral  substances  are  distinguished 
from  each  other  by  certain  external  characters,  such  as  the 

34  Jukes'  "Manual  of  Geology,"  p.  65.  35  Neues  Jahrb.  1882  (ii.)  p.  5. 

GEOLOGY— Vol.  XXIX— 7 


146  TEXT-BOOK   OF   GEOLOGY 

size,  form,  and  arrangement  of  their  component  particles. 
These  characters,  readily  perceptible  to  the  naked  eye,  and 
in  the  great  majority  of  cases  observable  in  hand  specimens, 
are  termed  megascopic  or  macroscopic  (pp.  146-156),  to  dis- 
tinguish them  from  the  more  minute  features  which,  being 
only  visible  or  satisfactorily  observable  when  greatly  mag- 
nified, are  known  as  microscopic  (pp.  161-172).  The  larger 
(geotectonic)  aspects  of  rock-structure,  which  can  only  be 
properly  examined  in  the  field  and  belong  to  the  general 
architecture  of  the  earth's  crust,  are  treated  of  in  Book  IV.3" 

In  the  discrimination  of  rocks,  it  is  not  enough  to  specify 
their  component  minerals,  for  the  same  minerals  may  con- 
stitute very  distinct  varieties  of  rock.  For  example,  quartz 
and  mica  form  the  massive  crystalline  rock,  greisen,  the 
foliated  crystalline  rock,  mica-schist,  and  the  sedimentary 
rock,  micaceous  sandstone.  Chalk,  encrinal  limestone,  sta- 
lagmite, statuary  marble  are  all  composed  of  calcite.  It  is 
needful  to  take  note  of  the  megascopic  and  microscopic 
structure  and  texture,  the  state  of  aggregation,  color,  and 
other  characters  of  the  several  masses. 

Four  methods  of  procedure  are  available  in  the  investi- 
gation and  determination  of  rocks:  1st,  megascopic  (macro- 
scopic) examination,  either  by  the  rough  and  ready,  but 
often  sufficient,  appliances  for  use  in  the  field,  or  by  those 
for  more  careful  work  indoors;  2d,  chemical  analysis;  3d, 
chemical  synthesis;  4th,  microscopic  investigation. 

i.   Megascopic  (Macroscopic)  Examination 
Tests  in  the  field. — The  instruments  indispensable  for  the 

36  The  student  who  would  pursue  physical  geology  by  original  research  in 
the  field  and  abroad  may  consult  Bone,  "Guide  du  Geologue  Voyageur,"  2  vols. 
1835;  6lie  de  Beaumont,  "Lecons  de  Geologic  pratique,"  vol.  i.  1845;  Penning 
and  Jukes-Brown.  "Field  Geology,"  2d  edit.  1880;  A.  Geikie,  "Outlines  of  Field 
Geology, "  4th.  edit.  1891.  F.  v.  Richthoferi,  "Fiihrer  fiir  Forschungsreisende," 
1886;  Grenville  Cole,  "Aids  in  Practical  Geology,"  1891. 


GEOGNOSY  14:7 

investigation  of  rocks  in  the  field  are  few  in  number,  and 
simple  in  character  and  application.  The  observer  will  be 
sufficiently  accoutred  if  he  carries  with  him  a  hammer  of 
such  form  and  weight  as  will  enable  him  to  break  off  clean, 
sharp,  un weathered  chips  from  the  edges  of  rock- masses, 
a  small  lens,  a  pocket-knife  of  hard  steel  for  determining 
the  hardness  of  rocks  and  minerals,  a  magnet  or  a  magne- 
tized knife-blade,  and  a  small  pocket-phial  of  dilute  hydro- 
chloric acid,  or  better  still  some  citric  acid  in  powder. 

Should  the  object  be  to  form  a  collection  of  rocks,  a 
hammer  of  at  least  three  or  four  pounds  in  weight  should 
be  carried:  also  one  or  two  chisels  and  a  small  trimming 
hammer,  weighing  about  \  lb.,  for  reducing  the  specimens 
to  shape.  A  convenient  size  of  specimen  is  4x3x1  inches. 
They  should  be  as  nearly  as  possible  uniform  in  size,  so 
as  to  be  capable  of  orderly  arrangement  in  the  drawers 
or  shelves  ol  a  case  or  cabinet.  Attention  should  be  paid 
not  only  to  obtain  a  thoroughly  fresh  fracture  of  a  rock,  but 
also  a  weathered  surface,  wherever  there  is  anything  char- 
acteristic in  the  weathering.  Every  specimen  should  have 
affixed  to  it  a  label,  indicating  as  exactly  as  possible  the 
locality  from  which  it  was  taken.  This  information  ought 
always  to  be  written  down  in  the  field  at  the  time  of  col- 
lecting, and  should  be  affixed  to  or  wrapped  up  with  the 
specimen,  before  it  is  consigned  to  the  collecting  bag.  If, 
however,  the  student  does  not  purpose  to  form  a  collection, 
but  merely  to  obtain  such  chips  as  will  enable  him  to  judge 
of  the  characters  of  rocks,  a-  hammer  weighing  from  1J  to  2 
Ibs.,  with  a  square  face  and  tapering  to  a  chisel- edge  at  the 
opposite  end,  will  be  most  useful.  The  advantage  of  this 
form  is  that  the  hammer  can  be  used  not  only  for  breaking 
hard  stones,  but  also  for  splitting  open  shales  and  other 
fissile  rocks,  so  that  it  unites  the  uses  of  hammer  and  chisel. 

It  is,  of  course,  desirable  that  the  learner  should  first 
acquire  some  knowledge  of  the  nomenclature  of  rocks,  b;y 
carefully  studying  a  collection  of  correctly  named  and  judi- 
ciously selected  rock-specimens.  Such  collections  may  now 
be  purchased  at  small  cost  from  mineral  dealers,  or  may  be 
studied  in  the  museums  of  most  towns.  Having  accustomed 
his  eye  to  the  ordinary  external  characters  of  rocks,  and  be- 
come familiar  with  their  names,  the  student  may  proceed 
to  determine  them  for  himself  in  the  field. 

Finding  himself  face  to  face  with  a  rock-mass,  and  after 
noting  its  geotectonic  characters  (Book  IV.),  the  observer 
will  proceed  to  examine  the  exposed  or  weathered  surface. 


148  TEXT-BOOK    OF   GEOLOGY 

The  earliest  lesson  he  has  to  learn,  and  that  of  which  per- 
haps he  will  in  after  life  meet  with  the  most  varied  illustra- 
tions, is  the  extent  to  which  weathering  conceals  the  true 
aspect  of  rocks.  From  what  has  been  said  in  previous 
pages,  the  nature  of  some  of  the  alterations  will  be  under- 
stood, and  further  information  regarding  the  chemical  proc- 
esses at  work  will  be  found  in  Book  III.  The  practical 
study  of  rocks  in  the  field  soon  discloses  the  fact,  that 
while,  in  some  cases,  the  weathered  crust  so  completely 
obscures  the  essential  character  of  a  rock  that  its  true 
nature  might  not  be  suspected,  in  other  instances,  it  is  the 
weathered  crust  that  best  reveals  the  real  structure  of  the 
mass.  Spheroidal  crusts  of  a  decomposing  yellow  ferrugi- 
nous earthy  substance,  for  example,  would  hardly  be  iden- 
tified as  a  compact  dark  basalt,  yet,  on  penetrating  within 
these  crusts,  a  central  core  of  still  undecomposed  basalt 
may  not  infrequently  be  discovered.  Again,  a  block  of 
limestone  when  broken  open  may  present  only  a  uniformly 
crystalline  structure,  yet  if  the  weathered  surface  be  exam- 
ined it  may  show  many  projecting  fragments  of  shells, 
polyzoa,  corals,  crinoids,  or  other  organisms.  The  really 
fossiliferous  nature  of  an  apparently  unfossiliferous  rock 
may  thus  be  revealed  by  weathering.  Many  limestones 
also  might,  from  their  fresh  fracture,  be  set  down  as  toler- 
ably pure  carbonate  of  lime;  but  from  the  thick  crust  of 
yellow  ochre  on  their  weathered  faces  are  seen  to  be  highly 
ferruginous.  Among  crystalline  rocks,  the  weathered  sur- 
face commonly  throws  light  upon  the  mineral  constitution 
of  the  mass,  for  some  minerals  decompose  more  rapidly  than 
others,  which  are  thus  left  isolated  and  more  easily  recog- 
nizable. In  this  manner,  the  existence  of  quartz  in  many 
felspathic  rocks  may  be  detected.  Its  minute  blebs  or  crys- 
tals, which  to  the  naked  eye  or  lens  are  lost  among  the 
brilliant  facets  of  the  felspars,  stand  out  amid  the  dull  clay 
into  which  these  minerals  are  decomposed. 

The  depth  to  which  weathering  extends  should  be  noted. 
The  student  must  not  be  too  confident  that  he  has  reached 
its  limit,  even  when  he  comes  to  the  solid,  more  or  less 
hard,  splintery,  and  apparently  fresh  stone.  Granite  some- 
times decomposes  into  Kaolin  and  sand  to  a  depth  of  twenty 
or  thirty  feet  or  more.  Limestones,  on  the  other  hand, 
have  often  a  mere  film  of  crust,  because  their  substance  is 
almost  entirely  dissolved  and  removed  by  rain  (Book  III. 
Part  II.  Section  ii.  §  2). 

With  some  practice,  the  inspection  of  a  weathered  sur- 


GEOGNOSY  149 

face  will  frequently  suffice  to  determine  the  true  nature 
and  name  of  a  rock.  Should  this  preliminary  examination, 
and  a  comparison  of  weathered  and  unweathered  surfaces, 
fail  to  afford  the  information  sought,  we  proceed  to  apply 
some  of  the  simple  and  useful  tests  available  for  field-work. 
The  lens  will  usually  enable  us  to  decide  whether  the  rock 
is  compact  and  apparently  structureless,  or  crystalline,  or 
fragmental.  Having  settled  this  point,  we  proceed  to  ascer- 
tain the  hardness  and  color  of  streak,  by  scratching  a  fresh 
surface  of  the  stone.  A  drop  of  acid  placed  upon  the 
scratched  surface  or  on  the  powder  of  the  streak  may  re- 
veal the  presence  of  some  carbonate.  By  practice,  consider- 
able facility  can  be  acquired  in  approximately  estimating 
the  specific  gravity  of  rocks  merely  by  the  hand.  The  fol- 
lowing table  may  be  of  assistance,  but  it  must  be  under- 
stood at  the  outset  that  a  knowledge  of  rocks  can  never 
be  gained  from  instructions  given  in  books,  but  must  be 
acquired  by  actual  handling  and  study  of  the  rocks  them- 
selves. 

i.  A  fresh  fracture  shows  the  rock  to  be  close-grained,  dull,  with  no 
distinct  structure.37 

«.  H.  0-5  or  less  up  to  1.  Soft,  crumbling  or  easily 
scratched  with  the  knife,  if  not  with  the  finger-nail; 
emits  an  earthy  smell  when  breathed  upon,  does  not 
effervesce  with  acid;  is  dark  gray,  brown,  or  blue, 
perhaps  red,  yellow,  or  even  white = probably  some 
clay  rock,  such  as  mudstone,  massive  shale,  or  fire- 
clay (p.  234);  or  a  decomposed  felspar-rock,  like  a 
close-grained  felsite  or  orthoclase  porphyry.  If  the 
rock  is  hard  and  fissile  it  may  be  shale  or  clay-slate 
(p.  235). 

/9.  H.  1-5-2.     Occurs  in  beds  or  veins  (perhaps  fibrous), 
white,  yellow,  or  reddish.     Sp.   gr.   2-2-2*4.     Does 
not  effervesce= probably  gypsum  (pp.  143,  265). 
Y.  Friable,  crumbling,  soils  the  fingers,  white,  or  yellow- 
ish, brisk  effervescence= chalk,  marl,  or  some  pul- 
verulent form  of  limestone  (pp.  244,  260). 
d.  H.  3-4.     Sp.  gr.  2-5-2-7.     Pale  to  dark  green  or  red- 
dish, or  with  blotched  and  clouded  mixtures  of  these 
colors.     Streak  white;  feels  soapy;  no  effervescence, 

31  In  this  table,  H.  =  hardness ;  Sp.  gr.  =  specific  gravity.  The  scale  of 
hardness  usually  employed  is  1,  Talc;  2,  Rock-salt  or  gypsum;  3,  Calcite; 
4,  Fluorite;  5,  Apatite;  6,  Orthoclase;  7,  Quartz;  8,  Topaz;  9,  Corundum; 
10,  Diamond. 


150  TEXT-BOOK  OF   GEOLOGY 

splintery  to  subconchoidal  fracture,  edges  subtrans- 
lucent.  See  serpentine  (p.  301). 

e.  H.  averaging  3.  Sp.  gr.  2-6-2-8.  White,  but  more 
frequently  bluish-gray,  also  yellow,  brown  and 
black;  streak  white;  gives  brisk  effervescence  = 
some  form  of  limestone  (pp.  244,  260). 

C.  H.  3-5^-5.  Sp.  gr.  2-8-2-95.  Yellowish,  white,  or 
pale  brown.  Powder  slowly  soluble  in  acid  with 
feeble  effervescence,  which  becomes  brisker  when 
the  acid  is  heated  with  the  powder  of  the  stone.  See 
dolomite  (pp.  143,  264). 

7.  H.  3^.  Sp.  gr.  3-3-9.  Dark  brown  to  dull  black, 
streak  yellow  to  brown,  feebly  soluble  in  acid,  which 
becomes  yellow;  occurs  in  nodules  or  beds,  usually 
with  shale;  weathers  with  browii  or  blood-red  crust 
=  brown  iron-ore.  See  clay-ironstone  (pp.  256,  267); 
and  limonite(pp.  128,  266);  if  the  rock  is  reddish  and 
gives  a  cherry-red  streak,  see  haematite  (pp.  128,  266). 

6.  Sp.  gr.  2-55.  White,  gray,  yellowish,  or  bluish,  rings 
under  the  hammer,  splits  into  thin  plates,  does  not 
effervesce,  weathered  crust  white  and  distinct=per- 
haps  some  compact  variety  of  phonolite  (p.  289.  See 
also  felsite,  p.  280,  and  porphyrite,  p.  292). 

e.  Sp.  gr.  2-9-3-2.  Black  or  dark  green,  weathered  crust 
yellow  or  brown=  probably  some  close-grained  va- 
riety of  basalt  (p.  296),  andesite  (p.  289),  aphanite 
(p.  288),  or  ampnibolite  (p.  314). 


*.  H.  6-6-5,  but  less  according  to  decomposition.  Sp. 
gr.  2-55-2-7.  Can  with  difficulty  be  scratched  with 
the  knife  when  fresh.  White,  bluish-gray,  yellow, 
lilac,  brown,  red;  white  streak;  sometimes  with  well 
denned  white  weathered  crust,  no  effervesce  nee  = 
probably  a  felsitic  rock  (p.  280). 

H.  H.  7.  Sp.  gr.  2-5-2-9.  The  knife  leaves  a  metallic 
streak  of  steel  upon  the  resisting  surface.  The  rock 
is  white,  reddish,  yellowish,  to  brown  or  black,  very 
finely  granular  or  of  a  horny  texture,  gives  no  reac- 
tion with  acid  =  probably  silica  in  the  form  of  jasper, 
hornstone,  flint,  chalcedony,  halleflinta  (pp.  127,  316), 
adinole  (p.  317). 

ii.  A  fresh  fracture  shows  the  rock  to  be  glassy. 

Leaving  out  of  account  some  glass-like  but  crystalline 
minerals,  such  as  quartz  and  rock-salt,  the  number  of  vitre- 
ous rocks  is  comparatively  small.  The  true  nature  of  the 
mass  in  question  will  probably  not  be  difficult  to  determine. 


GEOGNOSY  151 

It  must  be  one  of  the  Massive  volcanic  rocks  (p.  269  et  seq.}. 
If  it  occurs  in  association  with  siliceous  lavas  (liparites, 
trachytes)  it  will  probably  be  obsidian  (p.  282),  or  pitchstone 
(p.  283);  if  it  passes  into  one  of  the  basalt-rocks,  as  so  com- 
monly happens  along  the  edges  of  dikes  and  intrusive 
sheets,  it  is  a  glassy  form  of  basalt  (p.  297).  Each  of  the 
three  great  series  of  eruptive  rocks,  Acid,  Intermediate,  and 
Basic,  has  its  glassy  varieties  (see  pp.  282-284,  297). 
iii.  A  fresh  fracture  shows  the  rock  to  be  crystalline. 
If  the  component  crystals  are  sufficiently  large  for  deter- 
mination in  the  field,  they  may  suggest  the  name  of  the 
rock.  Where,  however,  they  are  too  minute  for  identifica- 
tion even  with  a  good  lens,  the  observer  may  require  to  sub- 
mit the  rock  to  more  precise  investigation  at  home,  before 
its  true  character  can  be  ascertained.  For  the  purposes  of 
field-work,  however,  the  following  points  should  be  noted, 
a.  The  rock  can  be  easily  scratched  with  the  knife. 

(a)  Effervesces  briskly  with  acid  =  limestone.  (5)  Pow- 
der of  streak  effervesces  in  hot  acid.  See  dolo- 
mite (p.  264).  (c)  No  effervescence  with  acid: 
may  be  granular  crystalline  gypsum  (alabaster) 
or  anhydrite  (pp.  143,  265). 

/J.  The  rock  is  not  easily  scratched.  It  is  almost  cer- 
tainly a  silicate.  Its  character  should  be  sought 
among  the  massive  crystalline  rocks  (p.  268).  If  it 
be  heavy,  appear  to  be  composed  of  only  one  min- 
eral, and  have  a  marked  greenish  tint,  it  may  be 
some  kind  of  amphibolite  (p.  314);  if  it  consist  of 
some  white  mineral  (felspar)  and  a  green  mineral 
which  gives  it  a  distinct  green  color,  while  the 
weathered  crust  shows  more  or  less  distinct  effer- 
vescence, it  may  be  a  fine-grained  diorite  (p.  286), 
or  diabase  (p.  296);  if  it  be  gray  and  granular,  with 
striated  felspars  and  dark  crystals  (augite  and  mag- 
netite), with  a  yellowish  or  brownish  weathered 
crust,  it  is  probably  a  dolerite  (p.  294)  or  andesite 
(p.  289);  if  it  be  compact,  finely-crystalline,  scratched 
with  difficulty,  showing  crystals  of  orthoclase,  and 
with  a  bleached  argillaceous  weathered  crust,  it  is 
probably  an  orthoclase-porphyry  (p.  285),  or  quartz- 
porphyry  (p.  278).  The  occurrence  of  distinct  blebs 
or  crystals  of  quartz  in  the  fresh  fracture  or  weath- 
ered face  will  suggest  a  place  for  the  rock  in  the 
quartziferous  crystalline  series  (granites,  quartz-por- 
phyries, rhyolites),  or  among  the  gneisses  and  schists. 


152  TEXT-BOOK   OF   GEOLOGY 

iv.  A  fresh  fracture  shows  the  rock  to  have  a  foliated  structure. 

The  foliated  rocks  are  for  the  most  part  easily  recogniz- 
able by  the  prominence  of  their  component  minerals  (p.  303). 
Where  the  minerals  are  so  intimately  mingled  as  not  to  be 
separable  by  the  use  of  the  leas,  the  following  hints  may 
be  of  service: 

a.  The  rock  has  an  unctuous  feel,  and  is  easily  scratched. 
It  may  be  talc-schist  (p.  315),  chlorite-schist  (p.  315), 
sericitic  mica-schist  (p.  319),  or  foliated  serpentine 


(p.  316). 
^he 


/9.  The  rock:  emits  an  earthy  smell  when  breathed  on,  is 
harder  than  those  included  in  a,  is  fine-grained,  dark- 
gray  in  color,  splits  with  a  slaty  fracture  and  con- 
tains perhaps  scattered  crystals  of  iron-pyrites  or 
some  other  mineral.  It  is  some  argillaceous-schist 
or  clay-slate,  the  varieties  of  which  are  named  from 
the  predominant  inclosed  mineral,  as  chiastolite- 
slate,  andalusite-schist,  ottrelite-schist,  etc.  (p.  309); 
if  it  has  a  silky  lustre  it  may  be  phyllite. 
f.  The  rock  is  composed  of  a  mass  of  ray-like  or  fibrous 
crystals  matted  together.  If  the  fibres  are  exceed- 
ingly fine,  silky,  and  easily  separable,  it  is  probably 
asbestos;  if  tney  are  coarser,  greenish  to  white, 
glassy,  and  hard,  it  is  probably  an  actinolite-schist 
(p.  314).  Many  serpentines  are  seamed  with  veins 
of  the  fine  silky  fibrous  variety  termed  chrysotile, 
which  is  easily  scratched. 

8.  The  rock  has  a  hardness  of  nearly  7,  and  splits  with 
some  difficulty  along  micaceous  folia.  It  is  proba- 
bly a  quartzose  variety  of  mica-schist,  quartz-schist, 
or  gneiss  (pp.  309,  317-319). 

e.  The  rock  shows  on  its  weathered  surface  small  parti- 
cles of  quartz  and  folia  of  mica  in  a  fine  decompos- 
ing base.  It  is  probably  a  fine-grained  variety  of 
mica-schist  or  gneiss. 

v.  A  fresh  fracture  shows  the  rock  to  have  a  fragmental  (clastic) 
structure. 

Where  the  component  fragments  are  large  enough  to  be 
seen  by  the  naked  eye  or  with  a  lens,  there  is  usually  little 
difficulty  in  determining  the  true  nature  and  proper  name 
of  the  rock.  Two  characters  require  to  be  specially  consid- 
ered— the  component  fragments  and  the  cementing  paste. 

1.  The  Fragments. — According  to  the  shape,  size,  and 
composition  of  the  fragments,  different  names  are  assigned 
to  clastic  rocks. 


GEOGNOSY  153 

a.  Shape. — If  the  fragments  are  chiefly  rounded,  the 
rock  may  be  sought  in  the  sand  and  gravel  series  (p.  224), 
while  if  they  are  large  and  angular,  it  may  be  classed  as  a 
breccia  (p.  230).  Some  mineral  substances,  however,  do  not 
acquire  rounded  outlines,  even  after  long-continued  attri- 
tion. Mica,  for  example,  splits  up  into  thin  Iamina3,  which 
may  be  broken  into  small  flakes  or  spangles,  but  never  be- 
come rounded  granules.  Other  minerals,  also,  which  have 
a  ready  cleavage,  are  apt  to  break  up  along  their  cleavage- 

E lanes,  and  thus  to  retain  angular  contours.  Gale-spar  is  a 
imiliar  example  of  this  tendency.  Organic  remains  com- 
posed of  this  mineral  (such  as  crinoids  and  echinoids)  may 
often  be  noticed  in  a  very  fragmentary  condition,  having 
evidently  been  subjected  to  long-continued  comminution. 
Yet  angular  outlines  and  fresh  or  little  worn  cleavage-sur- 
faces may  be  found  among  them.  Many  limestones  consist 
largely  of  sub-angular  organic  debris.  '  Angular  inorganic 
detritus  is  characteristic  of  volcanic  breccias  and  tuffs 
(p.  238}. 

/?.  Size. — Where  the  fragments  are  hard,  rounded,  or 
sub-angular  quartzose  grains,  the  size  of  a  pin's  head  or  less, 
the  rock  is  probably  some  form  of  sandstone  (p.  231). 
Where  they  range  up  to  the  size  of  a  pea,  it  may  be  a 
pebbly  sandstone,  fine  conglomerate  or  grit;  where  they 
vary  from  the  size  of  a  pea  to  that  of  a  walnut,  it  is  an 
ordinary  gravel  or  conglomerate;  where  they  range  up  to 
the  size  of  a  man's  head  or  larger,  it  is  a  coarse  shingle  or 
conglomerate.  A  considerable  admixture  of  sub-angular 
stones  makes  it  a  brecciated  conglomerate  or  breccia;  but 
where  the  materials  are  loosely  aggregated,  the  deposit  may 
be  some  kind  of  glacial  drift,  such  as  moraine-stuft  or  bowl- 
der-clay (p.  235).  Large  angular  and  irregular  blocks  are 
characteristic  of  coarse  volcanic  agglomerates  (p.  240). 

Y.  Composition. — In  the  majority  of  cases,  the  frag- 
ments are  of  o.uartz,  or  at  least  of  some  siliceous  and  endur- 
ing mineral.  Sandstones  consist  chiefly  of  rounded  quartz- 
grains  (p.  231).  Where  these  are  unmixed  with  other 
ingredients,  the  rock  is  sometimes  distinguished  as  a  quart- 
zose sandstone.  Such  a.  rock  when  indurated  becomes 
quartzite  (p.  311).  Among  the  quartz-grains,  minute  frag- 
ments of  other  minerals  may  be  observed.  When  any  one 
of  these  is  prominent,  it  may  give  a  name  to  the  variety  of 
sandstone,  as  felspathic,  micaceous  (p.  186).  Volcanic  tuffs 
and  breccias  are  characterized  by  the  occurrence  of  lapilli 
(very  commonlv  cellular)  of  the  lavas  from  the  explosion  of 


154  TEXT-BOOK    OF   GEOLOGY 

which  they  have  been  formed.  Among  inter-bedded  vol- 
canic rocks,  the  student  will  meet  with  some  which  he  may 
be  at  a  loss  whether  to  class  as  volcanic,  or  as  formed  of 
ordinary  sediment.  They  consist  of  an  intermixture  of  vol- 
canic detritus  with  sand  or  mud,  and  pass  on  the  one  side 
into  true  tuffs,  on  the  other  into  sandstones,  shales,  lime- 
stones, etc.  If  the  component  fragments  of  a  non-crystalline 
rock  give  a  brisk  effervescence  with  acid,  they  are  calcare- 
ous, and  the  rock  (most  likely  a  limestone,  or  at  least  of  cal- 
careous composition)  may  be  searched  for  traces  of  fossils. 

2.  The  Paste.— It  sometimes  happens  that  the  component 
fragments  of  a  clastic  rock  cohere  merely  from  pressure  and 
without  any  discoverable  matrix.  This  is  occasionally  the 
case  with  sandstone.  Most  commonly,  however,  there  is 
some  cementing  paste.  If  a  drop  of  weak  acid  produces 
effervescence  from  between  the  component  non-calcareous 
grains  of  a  rock,  the  paste  is  calcareous.  If  the  grains  are 
coated  with  a  red  crust  which,  on  being  bruised  between 
white  paper,  gives  a  cherry-red  powder,  the  cementing  ma- 
terial is  the  anhydrous  peroxide  of  iron.  A  dark  brown  or 
black  matrix  which  can  be  dissipated  by  heating  is  bitumi- 
nous. Where  the  component  grains  are  so  firmly  cemented 
in  an  exceedingly  hard  matrix  that  they  break  across  rather 
than  separate  from  each  other  when  the  stone  is  fractured, 
the  paste  is  probably  siliceous. 

Determination  of  Specific  Gravity. — The  student  will  find  this 
character  of  considerable  advantage  in  enabling  him  to  dis- 
criminate between  rocks.  He  may  acquire  some  dexterity 
in  estimating,  even  with  the  hand,  the  probable  specific 
gravity  of  substances;  but  he  should  begin  by  determin- 
ing it  with  a  balance.  Jolly's  spring  balance  is  a  simple 
and  serviceable  instrument  for  this  purpose.  It  consists  of 
an  upright  stem  having  a  graduated  strip  of  mirror  let  into 
it,  in  front  of  which  hangs  a  long  spiral  wire,  with  rests  at 
the  bottom  for  weighing  a  substance  in  air  and  in  water. 
For  most  purposes  it  is  sufficiently  accurate,  and  a  determi- 
nation can  be  made  with  it  in  the  course  of  a  few  minutes.38 
Another  and  more  convenient  instrument  has  been  invented 
by  W.  N.  "Walker,  consisting  -of  a  lever  graduated  into 
inches  and  tenths,  and  resting  on  a  knife-edge  stand,  on  one 
side  of  whjch  is  placed  a  movable  weight,  while  on  the  long 

88  Jolly's  spring  balance  can  be  obtained  through  any  optician  or  mineral 
dealer  from  Berberich,  of  Munich,  for  nine  florins  or  27s.  In  the  United  States 
it  is  manufactured  by  Geo.  Wade  &  Co.,  at  the  Hoboken  Institute. 


GEOGNOSY  155 

graduated  side  the  substance  to  be  weighed  is  suspended. 
This  instrument  has  the  advantage  of  not  being  so  liable  to 
get  out  of  order  as  other  contrivances.8' 

Mechanical  Analysis. — Much  may  be  learned  regarding  the 
composition  of  a  rock  by  reducing  it  to  powder.  In  the  case 
of  many  sandstones  and  clays  this  reduction  may  easily  be 
effected  by  drying  the  stone  and  crumbling  it  between  the 
fingers.  But  where  the  material  is  too  compact  for  such 
treatment  some  fragments  of  it  placed  within  folds  of  paper 
upon  a  surface  of  steel  may  be  reduced  to  powder  by  a  few 
smart  blows  of  a  hammer.  The  powder  can  be  sifted 
through  sieves  of  varying  degrees  of  fineness  and  the  sepa- 
rate fragments  may  be  picked  out  with  a  fine  brush  and  ex- 
amined with  a  lens.  If  they  are  dark  in  color  they  may  be 
placed  on  white  paper,  if  light-colored  they  are  more  readily 
observed  upon  a  black  paper.  Portions  of  this  powder  may 
be  carefully  washed  and  mounted  with  Canada  balsam  on 
glass,  as  in  the  way  described  below  for  microscopic  slices. 
In  this  way  the  constituent  minerals  of  many  crystalline 
rocks  may  be  isolated  and  studied  with  great  facility.  For 
purposes  of  comparison  specimens  of  the  rock-forming  min- 
erals should  be  procured  and  treated  in  a  similar  way.  A 
series  of  typical  preparations  of  the  powder  or  minute  frag- 
ments of  such  minerals  affords  to  the  student  an  admirable 
basis  from  which  to  start  in  his  study  of  the  crystallographic 
and  optical  characters  of  the  minerals  which  he  will  require 
to  identify  among  the  constituents  of  rocks. 

Another  method  of  isolating  the  several  components  of 
certain  rocks  is  by  washing  the  triturated  materials  in  water 
and  allowing  the  sediment  to  subside.  The  finer  and  lighter 
particles  may  be  drawn  off,  while  the  coarser  and  heavier 
grains  will  sink  according  to  their  respective  specific  gravi- 
ties, and  may  then  be  separated  and  collected.  This  may 
be  done  by  means  of  a  wide  tube  with  a  stop-cock  at  the 
bottom,  or  by  gently  washing  the  powder  with  water  on1  an 
inclined  surface,  when,  as  in  the  analogous  treatment  of 
veinstones  and  ores  in  mining,  the  particles  arrange  them- 
selves according  to  their  respective  gravities,  the  lightest 
being  swept  away  by  the  current. 

Magnetic  particles  may  be  extracted  with  a  magnet,  the 
end  of  which  is  preserved  from  contact  with  the  powder  by 

39  See  Geol.  Mag.  1883,  p.  109,  for  a  description  and  drawing  of  this  instru- 
ment, and  the  manner  of  using  it.  It  may  be  obtained  of  Lowden,  optician, 
Dundee,  and  How  &  Co.,  Farringdon  Street,  London.  Its  price  is  31s.  6d. 


156  TEXT-BOOK    OF   GEOLOGY 

being  covered  with  fine  tissue-paper.  An  electro-magnet 
will  at  once  withdraw  the  particles  of  minerals  which  con- 
tain far  too  little  iron  to  be  ordinarily  recognized  as  mag- 
netic; in  this  way  the  particles  of  a  ferruginous  magnesian 
mica  may  in  a  few  seconds  be  gathered  out  of  the  powder 
of  a  granite.40 

Where  the  difference  between  the  specific  gravity  of  the 
component  minerals  of  a  rock  is  slight,  they  may  "be  sepa- 
rated by  means  of  a  solution  of  given  density.  Mr.  E.  Son- 
stadt  proposed  the  use  of  a  saturated  solution  of  iodide  of 
mercury  in  iodide  of  potassium,  which  has  a  maximum  den- 
sity of  nearly  3*2.41  Kohrbach's  solution,  consisting  of 
iodide  of  mercury  and  iodide  of  barium,  has  a  density  of 
as  much  as  3'588.4S  More  serviceable  is  the  solution  of 
borotungstate  of  cadium,  with  a  density  of  8 '28,  proposed 
by  D.  Klein.43  The  powder  of  a  rock  being  introduced  into 
one  of  these  liquids,  those  particles  whose  specific  gravity- 
exceeds  that  of  the  liquid  will  sink  to  the  bottom,  while 
those  which  are  lighter  will  float.  This  process  allows  of 
the  separation  of  the  felspars  from  each  other,  and  at  once 
eliminates  the  heavy  minerals  such  as  hornblende,  augite, 
and  black  mica.  By  the  addition  of  water  or  other  liquid, 
as  the  case  may  be,  the  specific  gravity  may  be  reduced,  and 
different  solutions  of  given  density  may  be  employed  for  de- 
termining and  isolating  rock-constituents.  This  method  of 
analysis  is  important  in  affording  a  ready  means  of  separat- 
ing the  quartz  and  felspar  of  a  rock.44 

Hydrofluoric  acid  may  be  used  in  separating  the  mineral 
constituents  of  rocks.  The  rock  to  be  studied  is  reduced 
to  powder  and  introduced  gently  into  a  platinum  capsule 
containing  the  concentrated  acid.  During  the  consequent 
effervescence,  the  mixture  is  cautiously  stirred  with  a  plati- 
num spatula.  Some  minerals  are  converted  into  fluorides, 
others  into  fluosilicates,  while  some,  particularly  the  iron- 


40  Mem.  Acad.  des  Sci.  xxxii.  No.  11;  Fouque  and  Michel-Levy,  "Mineral- 
ogie  Micrographique, "  p.  115. 

41  Chem.  News,  xxix.  (1874),  p.  128.         44  Neues  Jahrb.  1883,  p.  186. 

43  Compt.   rend,  xciii.  (1881),  p.  318.     More  recently  R.  Brauus  has  intro- 
duced methylene  iodide,  which  gives  a  density  of  3 -33  and  is  diluted  with  ben- 
zole.    Neues  Jahrb.   1886,  ii.  p.   72.     See  also  J.  W.  Retgers,  op.   cit.   1889, 
ii.  p.  185. 

44  Fouque  and  Michel-Levy,  "Mineralogie  Micrographique,"  p.  171.     Thou- 
let,  Bull.  Soc.  Miii.  France,  ii.  (1879),  p.  17.     A  cheap  form  of  instrument  for 
isolating  minerals  by  means  of  heavy  solutions  is  described  by  Mr.  J.  W.  Evans, 
Geol.  Mag.  1891,  p.  67. 


GEOGNOSY  157 

magnesia  species,  remain  undissolved.  The  thick  jelly  of 
silica  and  alumina  is  removed  with  water,  and  the  crystal- 
line minerals  lying  at  the  bottom  can  then  be  dried  and 
examined.  By  arresting  the  solution  at  different  stages  the 
different  minerals  may  be  isolated.  This  process  is  admi- 
rably adapted  for  collecting  the  pyroxene  of  pyroxenic 
rocks." 

ii.    Chemical  Analysis 

The  determination  of  the  chemical  composition  of  rocks 
by  detailed  analysis  in  the  wet  way,  demands  an  acquaint- 
ance with  practical  chemistry  which  comparatively  few 
geologists  possess,  and  is  consequently  for  the  most  part 
left  in  the  hands  of  chemists,  who  are  not  geologists.  But 
as  some  theoretical  questions  in  geology  involve  a  consider- 
able knowledge  of  chemical  processes,  so  a  satisfactory 
analysis  of  rocks  is  best  performed  by  one  who  under- 
stands the  nature  of  the  geological  problems  on  which 
such  an  analysis  may  be  expected  to  throw  light.  As  a 
rule,  detailed  chemical  analysis  lies  out  of  the  sphere  of 
a  geologist's  work:  yet  the  wider  his  knowledge  of  chemical 
laws  and  methods  the  better.  He  should  at  least  be  able 
to  employ  with  accuracy  the  simpler  processes  of  chemical 
research. 

Treatment  with  Acid. — The  geologist's  accoutrements  for 
the  field  should  include  a  small  bottle  of  powdered  citric 
acid,  or  one  with  a  mineral  acid,  and  provided  with  a  glass 
stopper  prolonged  downward  into  a  point.  Dilute  hydro- 
chloric acid  has  been  commonly  employed;  but  H.  C. 
Bolton  proposed  in  1877  the  use  of  organic  acids  in  place 
of  the  usual  mineral  acids.  Citric  acid  is  particularly  ser- 
viceable for  the  purpose,  and  has  the  advantage  over  the 
mineral  acids  that  it  can  be  carried  in  powder,  and  a  strong 
solution  of  it  in  water  can  be  made  in  such  quantity  and 
at  such  time  as  may  be  required.  A  little  of  the  powder 
placed  with  the  point  of  a  Knife  on  a  surface  of  limestone 
and  moistened  with  a  drop  of  water  will  give  the  proper 
reaction.46 

When  a  drop  of  acid  gives  effervescence  upon  a  surface 
of  rock,  the  reaction  is  caused  by  the  liberation  of  bubbles 

45  Fouque  and  Michel-Levy,  op.  cit.  p.  116. 

46  Ann.  New  York  Acad.  Sci.  i.  (1879)  p.  1.     Chem.  News,  xxxvi.  xxxvii., 
xxxviii.,  xliii. 


158  TEXT-BOOK    OF   GEOLOGY 

of  carbon  dioxide,  as  this  oxide  is  replaced  by  the  more 
powerful  acid.  Hence  effervescence  is  an  indication  of  the 
presence  of  carbonates,  and  when  brisk  is  specially  char- 
acteristic of  calcium-carbonate.  Limestone  and  markedly 
calcareous  rocks  may  thus  at  once  be  detected.  By  the 
same  means,  the  decomposition  of  such  rocks  as  dolerite 
may  be  traced  to  a  considerable  distance  inward  from  the 
surface,  the  original  lime-bearing  silicate  of  the  rock  having 
been  decomposed  by  infiltrating  rain-water,  and  partially 
converted  into  carbonate  of  lime.  This  carbonate  being  far 
more  sensitive  to  the  acid-test  than  the  other  carbonates 
usually  to  be  met  with  among  rocks,  a  drop  of  weak  cold 
acid  suffices  to  produce  abundant  effervescence  even  from 
a  crystalline  face.  But  the  effervescence  becomes  much 
more  marked  if  we  apply  the  acid  to  the  powder  of  the 
stone.  For  this  purpose,  a  scratch  may  be  made  and  then 
touched  with  acid,  when  a  more  or  less  copious  discharge 
of  carbonic  acid  may  be  obtained,  where  otherwise  it  might 
appear  so  feebly  as  perhaps  even  to  escape  observation. 
Some  carbonates,  dolomite  for  example,  are  hardly  affected 
by  acid  until  it  is  heated.  This  is  done  by  placing  some 
fragments  of  the  substance  at  the  bottom  of  a  test-tube, 
covering  them  with  acid  and  applying  a  flame. 

It  is  a  convenient  method  of  roughly  estimating  the 
purity  of  a  limestone,  to  place  a  fragment  of  the  rock  in 
acid.  If  there  is  much  impurity  (clay,  sand,  oxide  of  iron, 
etc.),  this  will  remain  behind  as  an  insoluble  residue,  and 
may  then  be  further  tested  chemically,  or  examined  with 
the  microscope.  In  this  way  many  limestones  among  the 
crystalline  schists  may  be  dissolved,  in  acetic  acid,  leaving 
a  residue  of  pyroxenes,  amphiboles,  micas  or  other  silicates. 
Of  course  the  acid,  especially  if  strong  mineral  acid  is  em- 
ployed, may  attack  some  of  the  non-calcareous  constituents, 
so  that  it  cannot  be  concluded  that  the  residue  absolutely 
represents  everything  present  in  the  rock  except  the  car- 
bonate of  lime;  but  the  proportion  of  non-calcareous  matter 
so  dissolved  by  the  acid  will  usually  be  small. 

Further  chemical  processes. — A  thorough  chemical  anal- 
ysis of  a  rock  or  mineral  is  indispensable  for  the  elucida- 
tion of  its  composition.  But  there  are  several  processes  by 
which,  until  that  complete  analysis  has  been  made,  the 
geologist  may  add  to  his  knowledge  of  the  chemical  nature 
of  the  objects  of  his  study.  It  is  commonly  the  case  that 
minerals  about  which  he  may  be  doubtful  are  precisely 


GEOGNOSY  159 

those  which,  from  their  small  size,  are  most  difficult  of 
separation  from  the  rest  of  the  rock  preparatory  to  analyt- 
ical processes.  The  mineral  apatite,  for  example,  occurs 
in  minute  hexagonal  prisms,  which  on  cross-fracture  might 
be  mistaken  for  nepheline,  or  even  sometimes  for  quartz. 
If,  however,  a  drop  of  nitric  acid  solution  of  molybdate 
of  ammonia  be  placed  upon  one  of  these  crystals,  a  yellow 
precipitate  will  appear  if  it  be  apatite.  Nepheline,  which  is 
another  hexagonal  mineral  likewise  abundant  in  some  rocks, 
gives  no  yellow  precipitate  with  the  ammonia  solution,  while 
if  a  drop  of  hydrochloric  acid  be  put  over  it,  crystals  of 
chloride  of  sodium  or  common  salt  will  be  obtained.  These 
reactions  can  be  observed  even  with  minute  crystals  or  frag- 
ments, by  placing  them  on  a  glass  slide  under  the  micro- 
scope and  using  an  exceedingly  attenuated  pipette  for  drop- 
ping the  liquid  on  the  slide.** 

Two  ingenious  applications  of  chemical  processes  to  the 
determination  of  minute  fragments  of  minerals  are  now  in 
use.  In  one  of  these,  devised  by  Boricky,48  hydrofluosilicic 
acid  of  extreme  purity  is  employed.  This  acid  decomposes 
most  silicates,  and  forms  from  their  bases  hydrofluosilicates. 
A  particle  about  the  size  of  a  pin's  head  of  the  mineral  to 
be  examined  is  fixed  by  its  base  upon  a  thin  layer  of  Canada 
balsam  spread  upon  a  slip  of  glass,  and  a  drop  of  the  acid  is 
placed  upon  it.  The  preparation  is  then  set  in  moist  air 
near  a  saucer  of  water  under  a  bell-glass  for  twenty-four 
hours,  after  which  it  is  inclosed  in  dry  air,  with  chloride  of 
calcium.  In  a  few  hours  the  hydrofluosilicates  crystallize 
out  upon  the  balsam  and  can  be  examined  with  the  micro- 
scope. Those  of  potassium  take  the  form  of  cubes,  of 
sodium  hexagonal  prisms,  etc. 

The  second  process,  devised  by  Szabo,  consists  in  utiliz- 
ing the  colorations  given  to  the 'flame  of  a  Bunseu-burner 
by  sodium  and  potassium.  An  elongated  splinter  of  the 
mineral  to  be  examined  is  first  placed  in  the  outer  or  oxi- 
dizing part  of  the  flame  near  the  base,  and  then  in  the  re- 

47  An  excellent  treatise  on  the  chemical  examination  of  minerals  under  the 
microscope  is  that  by  MM.  Klement  and  Renard,  "Reactions  microchemiques  A 
cristaux  et  leur  application  en  analyse  qualitative,"  Brussels,  1886.     See  also 
H.  Behrens,  Ann.  ficole  Polytechnique  de  Delft,  i.  1885,  p.  176;  Neues  Jahrb. 
vii.  Beilage  Band.  p.  435;  Zeitsch.  f.  Analyt.  Chemie,  xxx.  ii.  p.  126-174  (1891). 

48  Archiv  Naturwiss.  Landesdurchforschung  von  Bohmen,  iii.  fasc.  3,  1876. 
"Elemente  einer  neuen  chemisch-mikroskopischen  Mineral-  und  Gesteinsana- 
lyae."     Prag.  1877. 


TEXT-BOOK   OF   GEOLOGY 

ducing  part  further  up  and  nearer  the  centre.  The  amount 
of  sodium  present  in  the  mineral  is  indicated  by  the  extent 
to  which  the  flame  is  colored  yellow.  The  potassium  is 
similarly  estimated,  but  the  flame  is  then  looked  at  with 
cobalt  glass,  so  as  to  eliminate  the  influence  of  the  sodium.4' 
Blow-pipe  Tests. — The  chemical  tests  with  the  blow-pipe 
are  simple,  easily  applied,  and  require  only  patience  and 
practice  to  give  great  assistance  in  the  determination  of 
minerals,  if  unacquainted  with  blow-pipe  analysis,  the 
student  must  refer  to  one  or  other  of  the  numerous  text- 
books on  the  subject,  some  of  which  are  mentioned  below.80 
For  early  practice  the  following  apparatus  will  be  found 
sufficient: 

1.  Blow-pipe. 

2.  Thick-wicked   candle,   or   a  tin   box   filled  with  the 
material  of  Child's  night-lights,  and  furnished  with  a  piece 
of  Freyberg  wick  in  a  metallic  support. 

3.  "Platinum-tipped  forceps. 

4.  A  few  pieces  of  platinum  wire  in  lengths  of  three  or 
four  inches. 

5.  A  few  pieces  of  platinum  foil. 

6.  Some  pieces  of  cnarcoal. 

7.  A  number  of  closed  and  open  tubes  of  hard  glass. 

8.  Three   small    stoppered    bottles    containing    sodium- 
carbonate,  borax,  and  microcosmic  salt. 

9.  Magnet. 

This  list  can  be  increased  as  experience  is  gained.  The 
whole  apparatus  may  easily  be  packed  into  a  box  which  will 
go  into  the  corner  of  a  portmanteau. 

iii.    Chemical  Synthesis 

As  already  remarked  (p.  118),  much  interesting  light  has 
been  thrown  on  the  natural  conditions  in  which  minerals 


49  Szabo,   "Deber  eine  neue  Methods  die  Felspathe  auch  in  Gesteinen  zu 
bestimmen."     Buda-Pesth,  1876. 

50  The  great  work  on  the  blow-pipe  is  Plattner's,  of  which  an  English  trans- 
lation has  been   published.      Elderhorst's   "Manual  of  Qualitative  Blow-pipe 
Analysis  and  Determinative  Mineralogy,"  by  H.  B.  Nason  and  C.  F.  Chandler 
(Philadelphia:  N.  S.  Porter  and  Coates),  is  a  smaller  but  useful  volume;  while 
still  less  pretending  is  Scheerer's  "Introduction  to  the  Use  of  the  Mouth  Blow- 
pipe," of  which  a  third  edition  by  H.  F.  Blandford  was  published  in  1875  by 
F.   Norgate.     An  admirable  work  of  reference  will  be  found  in  Prof.  Brush's 
"Manual  of  Determinative  Mineralogy"  (New  York:  J.  Wiley  and  Son).     F.  v. 
KobelPs  "Tafeln  zur  Bestimmung  der  Mineralien"  (Munich)  are  useful.     A  valu- 
able summary  will  be  found  in  Prof.  Cole's  "Aids  in  Practical  Geology,"  1891. 


GEOGNOSY  161 

and  rocks  have  been  formed,  by  actual  experiments  in 
which  these  bodies  are  reproduced  artificially.  Since  the 
classic  experiments  of  Hall  much  progress  has  been  made 
in  this  subject,  notably  from  the  prolonged  and  admirable 
researches  carried  on  in  Paris  by  Prof.  Dauhre'e  and  by 
Messrs.  Fouque"  and  Michel-LeVy.  To  some  of  the  results 
obtained  by  these  observers  reference  will  be  made  in  Book 
III.  Part  I.  Sect.  iv.  The  processes  of  investigation  have 
been  grouped  in  three  classes.  1st.  Those  by  the  "dry 
way"  as  in  fusion  and  sublimation,  sometimes  simply, 
sometimes  with  the'  intervention  of  a  mineralizing  agent 
such  as  borax,  borates,  fluorides,  chlorides,  etc.  2d.  Those 
by  the  "wet  way"  where  water  or  steam  are  used  as  dis- 
solvents either  by  themselves  or  with  the  aid  of  some 
mineralizing  agent;  and  3d,  Those  where  some  combina- 
tion of  the  two  foregoing  methods  is  employed,  that  is, 
where  water  or  steam  is  made  to  act  at  a  high  temperature 
and  under  great  pressure." 

iv.  Microscopic  Investigation" 

The  value  of  the  microscope  as  an  aid  in  geological  re- 
search is  now  everywhere  acknowledged.  Some  informa- 
tion may  here  be  given  as  to  the  methods  of  procedure  in 
microscopical  inquiry. 

1.  Preparation  of  microscopic  slides  of  rocks  and  minerals. — The 
observer  ought  to  be  able  to  prepare  his  own  slices,  and  in 
many  cases  will  find  it  of  advantage  to  do  so,  or  at  least 
personally  to  superintend  their  preparation  by  others.  It 
is  desirable  that  he  should  know  at  the  outset  that  no  costly 
or  unwieldy  set  of  apparatus  is  needful  for  his  purpose.  If 
he  is  resident  in  one  place  and  can  accommodate  a  cutting 
machine  such  as  a  lapidary's  lathe,  he  will  find  the  process 

51  See  on  this  subject  Daubree's  great  work  "Geologie  Experimentale, " 
1879;  Fouque  et  Michel-Levy,  "Synthese  des  Mine>aux  et  des  Roches,"  1882; 
Stanislas  Meunier,  "Les  M&hodes  de  Synthese  en  MineValogie, "  1891;  also 
postea,  p.  513  et  seq. 

5i  On  the  microscopic  investigation  of  rocks  consult  Fouque  and  Michel-Levy, 
"MineralogieMicrographique,"  2  vols.  Paris,  1879;  Michel -Levy,  "Les  Mineraux 
des  Roches, ' '  Paris,  1888 ;  Michel-Levy  and  Lacroix,  "Tableaux  des  Mineraux  des 
Roches,"  1889;  Rosenbusch,  "Mikroskopische  Physiographic  der  Mineralien 
und  Gesteine,"  2  vols.,  one  of  which  has  been  translated  into  English  by 
Iddinga  and  published  by  Macmillan  &  Co. ;  also  his  "Hulfstabellen  zur  Mikro- 
skopischen  Mineralbestimmung,"  1888,  translated  into  English  by  F.  H.  Hatch 
aud  published  by  Swan  Som;enschein  &  Co. ;  F.  Rutley,  "Rock-forming  Min- 
erals," London,  1888,  and  Prof.  Cole's  volume  above  cited. 


162  TEXT-BOOK   OF   GEOLOGY 

of  preparing  rock-slices  greatly  facilitated."  The  thickness 
of  each  slice  must  be  mainly  regulated  by  the  nature  of  the 
rock,  the  rule  being  to  make  the  slice  as  thin  as  can  con- 
veniently be  cut,  so  as  to  save  labor  in  grinding  down 
afterward.  Perhaps  the  thickness  of  a  shilling  may  be 
taken  as  a  fair  average.  The  operator,  however,  may  still 
further  reduce  this  thickness  oy  cutting  and  polishing 
a  face  of  the  specimen,  cementing  that  on  glass  in  the  way 
to  be  immediately  described,  and  then  cutting  as  close  as 
possible  to  the  cemented  surface.  The  thin  slice  thus  left  on 
the  glass  can  then  be  ground  down  with  comparative  ease. 

Excellent  rock-sections,  however,  may  be  prepared  with- 
out any  machine,  provided  the  operator  possesses  ordinary 
neatness  of  hand  and  patience.  He  must  procure  as  thin 
chips  as  possible.  Should  the  rocks  be  accessible  to  him 
in  the  field,  he  should  select  the  freshest  portions  of  them, 
and  by  a  dexterous  use  of  the  hammer,  break  off  from 
a  sharp  edge  a  number  of  thin  splinters  or  chips,  out  of 
which  he  can  choose  one  or  more  for  rock-slices.  These 
chips  may  be  about  an  inch  square.  It  is  well  to  take  sev- 
eral of  them,  as  the  first  specimen  may  chance  to  be  spoiled 
in  the  preparation.  The  geologist  ought  also  always  to  carry 
off  a  piece  of  the  same  block  from  which  his  chip  is  taken, 
that  he  may  have  a  specimen  of  the  rock  for  future  reference 
and  comparison.  Every  such  hand-specimen,  as  well  as  the 
chips  belonging  to  it,  ought  to  be  wrapped  up  in  paper  on 
the  spot  where  it  is  obtained,  and  with  it  should  be  placed 
a  label  containing  the  name  of  the  locality  and  any  notes 
that  may  be  thought  necessary.  It  can  hardly  be  too  fre- 
quently reiterated  that  all  such  field-notes  ought  as  far  as 
possible  to  be  written  down  on  the  ground,  when  the  actual 
facts  are  before  the  eye  for  examination. 

53  A.  machine  well  adapted  for  both  cutting  and  polishing  was  devised  some 
years  ago  by  Mr.  J.  B.  Jordan,  and  may  be  had  of  Messrs.  Cotton  and  John- 
son, Gerrard"  Street,  Soho,  London,  for  £10  10s.  Another  slicing  and  polishing 
machine,  invented  by  Mr.  F.  G.  Cuttell,  costs  £6  10s.  These  machines  are  too 
unwieldy  to  be  carried  about  the  country  by  a  field-geologist.  Fuess  of  Berliu 
supplies  two  small  and  convenient  hand-instruments,  one  for  slicing,  the  other 
for  grinding  and  polishing.  The  slicing-machine  is  not  quite  so  satisfactory 
for  hard  rocks  as  one  of  the  larger,  more  solid  forms  of  apparatus  worked  by 
a  treadle.  But  the  grinding-machine  is  useful,  and  might  be  added  to  a  geolo- 
gist's outfit  without  material  inconvenience.  If  a  lapidary  is  within  reach, 
much  of  the  more  irksome  part  of  the  work  may  be  saved  by  getting  him  to 
cut  off  the  thin  slices  in  directions  marked  for  him  upon  the  specimens.  Many 
lapidaries  now  undertake  the  whole  labor  of  cutting  and  mounting  microscopic 
slides. 


GEOGNOSY  168 

Having  obtained  his  thin  slices,  either  by  having  them 
slit  with  a  machine  or  by  detaching  with  a  hammer  as  thin 
splinters  as  possible,  the  operator  may  proceed  to  the  prepa- 
ration of  them  for  the  microscope.  For  this  purpose  the 
following  simple  apparatus  is  all  that  is  absolutely  needful, 
though  if  a  grinding-machine  be  added  it  will  save  time 
and  Tabor. 

List  of  Apparatus  required  in  the  Preparation  of  Thin  Slices 
of  Rocks  and  Minerals  for  Microscopical  Examination 

1.  A  cast-iron  plate  \  inch  thick  and  9  inches  square. 

2.  Two  pieces  of  plate-glass,  9  inches  square. 

3.  A  Water  of  Ayr  stone,  6  inches  long  by  2*  inches 
broad. 

4.  Coarse  emery  (1  Ib.  or  so  at  a  time). 

5.  Fine  or  flour-emery  (ditto). 

6.  Putty  powder  (1  oz.). 

7.  Canada  balsam.     (There  is  an  excellent  kind  prepared 
by  Rimmington,  Bradford,  specially  for  microscopic  prepa- 
rations, and  sold  in  shilling  bottles.) 

8.  A  small  forceps,  and  a  common  sewing-needle  with 
its  head  fixed  in  a  cork. 

9.  Some  oblong  pieces   of    common    flat  window-glass; 
2x1  inches  is  a  convenient  size. 

10.  Glasses  with  ground  edges  for  mounting  the  slices 
upon.     They  may  be  had  at  any  chemical  instrument  mak- 
er's in  different  sizes,  the  commonest  in  this  country  being 
3x1  inches,  though  this  size  is  rather  too  long  for  conven- 
ient handling  on  a  rotating  stage. 

11.  Thin  covering-glasses,  square  or  round.     These  are 
sold  by  the  ounce;  J  oz.  will  be  sufficient  to  begin  with. 

12.  A  small  bottle  of  spirits  of  wine. 

The  first  part  of  the  process  consists  in  rubbing  down 
and  polishing  one  side  of  the  chip  or  slice,  if  this  has  not 
already  been  done  in  cutting  off  a  slice  affixed  to  glass,  as 
above  mentioned.  We  place  the  chip  upon  the  wheel  of  the 
grinding-machine,  or,  failing  that,  upon  the  iron  plate,  with 
a  little  coarse  emery  and  water.  If  the  chip  is  so  shaped 
that  it  can  be  conveniently  pressed  by  the  finger  against  the 
plate  and  kept  there  in  regular  horizontal  movement,  we 
may  proceed  at  once  to  rub  it  down.  If,  however,  we  find 
a  difficulty,  from  its  small  size  or  otherwise,  in  holding  the 
chip,  one  side  of  it  may  be  fastened  to  the  end  of  a  bobbin 


164  TEXT-BOOK    OF   GEOLOGY 

or  other  convenient  bit  of  wood  by  means  of  a  cement 
formed  of  three  parts  of  resin  and  one  of  beeswax,  which 
is  easily  softened  by  heating.  A  little  practice  will  show 
that  a  slow,  equable  motion  with  a  certain  steady  pressure  is 
most  effectual  in  producing  the  desired  flatness  of  surface. 
When  all  the  roughnesses  have  been  removed,  which  can  be 
told  after  the  chip  has  been  dipped  in  water  so  as  to  remove 
the  mud  and  emery,  we  place  the  specimen  upon  the  square 
of  plate-glass,  and  with  flour-emery  and  water  continue  to 
rub  it  down  until  all  the  scratches  caused  by  the  coarse 
emery  have  been  removed  and  a  smooth  polished  surface 
has  been  produced.54  Care  should  be  taken  to  wash  the 
chip  entirely  free  of  any  grains  of  coarse  emery  before  the 
polishing  on  glass  is  begun.  It  is  desirable  also  to  reserve 
the  glass  for  polishing  only.  The  emery  gets  finer  and  finer 
the  longer  it  is  used,  so  that  by  remaining  on  the  plate  it 
may  be  used  many  times  in  succession.  Of  course  the  glass 
itself  is  worn  down,  but  by  using  alternately  every  portion 
of  its  surface  and  on  both  sides,  one  plate  may  be  made  to 
last  a  considerable  time.  If  after  drying  and  examining  it 
carefully,  we  find  the  surface  of  the  chip  to  be  polished  and 
free  from  scratches,  we  may  advance  to  the  next  part  of  the 
process.  But  it  will  often  happen  that  the  surface  is  still 
finely  scratched.  In  this  case  we  may  place  the  chip  upon 
the  Water  of  Ayr  stone  and  with  a  little  water  gently  rub  it 
to  and  fro.  It  should  be  held  quite  flat.  The  Water  of 
Ayr  stone,  too,  should  not  be  allowed  to  get  worn  into  a 
hollow,  but  should  also  be  kept  quite  flat,  otherwise  we 
shall  lose  part  of  the  chip.  Some  soft  rocks,  however,  will 
not  take  an  unscratched  surface  even  with  the  Water  of  Ayr 
stone.  These  may  be  finished  with  putty  powder,  applied 
with  a  bit  of  woollen  rag. 

The  desired  flatness  and  polish  having  been  secured,  and 
all  trace  of  scratches  and  airt  having  been  completely  re- 
moved, we  proceed  to  a  further  stage,"  which  consists  in 
grinding  down  the  opposite  side  and  reducing  the  chip  to 
the  requisite  degree  of  thinness.  The  first  step  is  now  to 
cement  the  polished  surface  of  the  chip  to  one  of  the  pieces 
of  common  glass.  A  thin  piece  of  iron  (a  common  shovel 


some 
pour- 


M  Exceedingly  impalpable  emery  powder  may  be  obtained  by  stirring 
of  the  finest  emery  in  water,  and  after  the  coarse  particles  have  subsided, 
ing  off  the  liquid  and  allowing  the  fine  suspended  dust  gradually  to  subsida 
Filtered  and  dried,  the  residue  can  be  kept  for  the  more  delicate  parts  of  the 
polishing. 


GEOGNOSY  165 

does  quite  well)  is  heated  over  a  fire,  or  is  placed  between 
two  supports  over  a  gas-flame."  On  this  plate  must  be  laid 
the  piece  of  glass  to  which  the  slice  is  to  be  affixed,  together 
with  the  slice  itself.  A  little  Canada  balsam  is  dropped  on 
the  centre  of  the  glass  and  allowed  to  remain  until  it  has  ac- 
quired the  necessary  consistency.  To  test  this  condition, 
the  point  of  a  knife  should  be  inserted  into  the  balsam,  and 
on  being  removed  should  be  rapidly  cooled  by  being  pressed 
against  some  cold  surface.  If  it  soon  becomes  hard  enough 
to  resist  the  pressure  of  the  finger  nail,  it  has  been  suffi- 
ciently heated.  Care,  however,  must  be  observed  not  to  let 
it  remain  too  long  on  the  hot  plate;  for  it  will  then  become 
brittle  and  start  from  the  glass  at  some  future  stage,  or  at 
least  will  break  away  from  the  edges  of  the  chip  and  leave 
them  exposed  to  the  risk  of  being  frayed  off.  The  heat 
should  be  kept  as  moderate  as  possible,  for  if  it  becomes  too 
great  it  may  injure  some  portions  of  the  rock.  Chlorite,  for 
example,  is  rendered  quite  opaque  if  the  heat  is  so  great  as 
to  drive  off  its  water. 

When  the  balsam  is  found  to  be  ready,  the  chip,  which 
has  been  warmed  on  the  same  plate,  is  lifted  with  the  for- 
ceps, and  laid  gently  down  upon  the  balsam.  It  is  well  to 
let  one  end  touch  the  balsam  first,  and  then  gradually  to 
lower  the  other,  as  in  this  way  the  air  is  driven  out.  With 
the  point  of  a  needle  or  a  knife  the  chip  should  be  moved 
about  a  little,  so  as  to  expel  any  bubbles  of  air  and  promote 
a  firm  cohesion  between  the  glass  and  the  stone.  The  glass 
is  now  removed  with  the  forceps  from  the  plate  and  put 
upon  the  table,  and  a  lead  weight  or  other  small  heavy  ob- 
ject is  placed  upon  the  chip,  so  as  to  keep  it  pressed  down 
until  the  balsam  has  cooled  and  hardened.  If  the  operation 
has  been  successful,  the  slide  ought  to  be  ready  for  further 
treatment  as  soon  as  the  balsam  has  become  cold.  If,  how- 
ever, the  balsam  is  still  soft,  the  glass  must  be  again  placed 
on  the  plate  and  gently  heated,  until  on  cooling,  the  balsata. 
fulfils  the  condition  of  resisting  the  pressure  of  the  finger- 
nail. 

Having  now  produced  a  firm  union  of  the  chip  and  the 
glass,  we  proceed  to  rub  down  the  remaining  side  of  the 
stone  with  coarse  emery  on  the  iron  plate  as  before.  If 
the  glass  cannot  be  held  in  the  hand  or  moved  by  the  sim- 

"  A  piece  of  wire-gauze  placed  over  the  flame,  with  an  interval  of  an  inch 
or  more  between  it  and  the  overlying  thin  iron  plate,  tends  to  diffuse  the  heat 
«-nd  prevail:  th?  ^wlsam  fi-om  being  unequally  heated. 


166  TEXT-BOOK    OF    OEOLOGY 

pie  pressure  of  i,be  fingers,  which  usually  suffices,  it  may  be 
fastened  to  the  end  of  the  bobbin  with  the  cement  as  before. 
When  the  chip  has  been  reduced  until  it  is  tolerably  thin; 
until,  for  example,  light  appears  through  it  when  held  be- 
tween the  eye  and  the  window,  we  may,  as  before,  wash  it 
clear  of  the  coarse  emery  and  continue  the  reduction  of  it  on 
the  glass  plate  with  fine  emery.  Crystalline  rocks,  such 
as  granite,  gneiss,  diorite,  dolerite,  and  modern  lavas,  can 
be  thus  reduced  to  the  required  thinness  on  the  glass  plate. 
Softer  rocks  may  require  gentle  treatment  with  the  Water 
of  Ayr  stone. 

The  last  parts  of  the  process  are  the  most  delicate  of  all. 
We  desire  to  make  the  section  as  thin  as  possible,  and  for 
that  purpose  continue  rubbing  until  after  one  final  attempt 
we  may  perhaps  find  to  our  dismay  that  great  part  of  the 
slice  has  disappeared.  The  utmost  caution  should  be  used. 
The  slide  should  be  kept  as  flat  as  possible,  and  looked  at 
frequently,  that  the  first  indications  of  disruption  may  be 
detected.  The  thinness  desirable  or  attainable  depends  in 
great  measure  upon  the  nature  of  the  rock.  Transparent 
minerals  need  not  be  so  much  reduced  as  more  opaque  ones. 
Some  minerals,  indeed,  remain  absolutely  opaque  to  the 
last,  like  pyrite,  magnetite,  and  ilmenite. 

The  slide  is  now  ready  for  the  microscope.  It  ought  al- 
ways to  be  examined  with  that  instrument  at  this  stage. 
We  can  thus  see  whether  it  is  thin  enough,  and  if  any 
chemical  tests  are  required  they  can  readily  be  applied  to 
the  exposed  surface  of  the  slice.  If  the  rock  has  proved 
to  be  very  brittle,  and  we  have  only  succeeded  in  procuring 
a  thin  slice  after  much  labor  and  several  failures,  nothing 
further  should  be  done  with  the  preparation,  unless  to  cover 
it  with  glass,  as  will  be  immediately  explained,  which  not 
only  protects  it,  but  adds  to  its  transparency.  But  where 
the  slice  is  not  so  fragile,  and  will  bear  removal  from  its 
original  rough  scratched  piece  of  glass,  it  should  be  trans- 
ferred to  one  of  the  glass-slides  (No.  10).  For  this  purpose, 
the  preparation  is  once  more  placed  on  the  warm  iron  plate, 
and  close  alongside  of  it  is  put  one  of  the  pieces  of  glass 
which  has  been  carefully  cleaned,  and  on  the  middle  of 
which  a  little  Canada  balsam  has  been  dropped.  The  heat 
gradually  loosens  the  cohesion  of  the  slice,  which  is  then 
very  gently  pushed  with  the  needle  or  knife  along  to  the 
contiguous  clean  slip  of  glass.  Considerable  practice  is 
needed  in  this  part  of  the  work,  as  the  slice,  being  so  thin, 
is  apt  to  go  to  pieces  in  being  transferred.  A  gentle  incli- 


GEOGNOSY  167 

nation  of  the  warm  plate,  so  that  a  tendency  may  be  given 
to  the  slice  to  slip  downward  of  itself  on  to  the  clean  glass, 
may  be  advantageously  given.  We  must  never  attempt  to 
lift  the  slice.  All  shifting  of  its  position  should  be  per- 
formed with  the  point  of  the  needle  or  other  sharp  instru- 
ment. If  it  goes  to  pieces  we  may  yet  be  able  to  pilot  the 
fragments  to  their  resting-place  on  the  balsam  of  the  new 
glass,  and  the  resulting  slide  may  be  sufficient  for  the  re- 
quired purpose. 

When  the  slice  has  been  safely  conducted  to  the  centre 
of  the  glass  slip,  we  put  a  little  Canada  balsam  over  it,  and 
warm  it  as  before.  Then  taking  one  of  the  thin  cover- 
glasses  with  the  forceps,  we  allow  it  gradually  to  rest  upon 
the  slice  by  letting  down  first  one  side,  and  then  by  degrees 
the  whole.  A  few  gentle  circular  movements  of  the  cover- 
glass  with  the  point  of  the  needle  or  forceps  may  be  needed 
to  insure  the  total  disappearance  of  air-bubbles.  When 
these  do  not  appear,  and  when,  as  before,  we  find  that  the 
balsam  has  acquired  the  proper  degree  of  consistence, 
the  slide  containing  the  slice  is  removed,  and  placed  on 
the  table  with  a  small  lead  weight  above  it  in  the  same 
way  as  already  described.  On  becoming  quite  cold  and 
hard  the  superabundant  balsam  round  the  edge  of  the  cover- 
glass  may  be  scraped  off  with  a  knife,  and  any  which  still 
adheres  to  the  glass  may  be  removed  with  a  little  spirits  of 
wine.  Small  labels  should  be  kept  ready  for  affixing  to  the 
slides  to  mark  localities  and  reference  numbers.  Thus 
labelled,  the  slide  may  be  put  away  for  future  study  and 
comparison. 

The  whole  process  seems  perhaps  a  little  tedious.  But 
in  reality  much  of  it  is  so  mechanical,  that  after  the  mode 
of  manipulation  has  been  learned  by  a  little  experience,  the 
rubbing-down  may  be  done  while  the  operator  is  reading. 
Thus  in  the  evening,  when  enjoying  a  pleasant  book  after 
his  day  in  the  field,  he  may  at  the  same  time,  after  som'e 
practice,  rub  down  his  rock-chips,  and  thus  get  over  the 
drudgery  of  the  operation  almost  unconsciously. 

Boxes,  with  grooved  sides  or  with  flat  trays  for  carrying 
microscopic  slides,  are  sold  in  different  sizes.  Such  boxes 
are  most  convenient  for  a  travelling  equipage,  as  they  go 
into  small  space,  and  with  the  help  of  a  little  cotton-wool 
they  hold  the  glass  slides  firmly  without  the  risk  of  break- 
age. For  a  final  resting-place,  a  case  with  shallow  trays  or 
drawers  in  which  the  slides  can  lie  flat  is  most  convenient. 

2.  The  Microscope. — Unless  the  observer  proposes  to  enter 


168  TEXT-BOOK    OF   GEOLOGY 

into  great  detail  in  the  investigation  of  the  minuter  parts  of 
rock  structure,  he  does  not  require  a  large  and  expensive  in- 
strument. For  most  geological  purposes,  objectives  of  2,  1, 
and  J  inch  focal  length  are  sufficient.  But  it  is  desirable 
also  for  special  work,  such  as  the  investigation  of  crystal- 
lites and  inclusions  of  minerals,  to  have  an  objective  capable 
of  magnifying  up  to  200  or  300  diameters.  An  instrument 
with  fairly  good  glasses  of  these  powers,  according  to  the 
arrangement  of  object-glasses  and  eye-pieces,  may  be  had  of 
some  London  makers  for  £5.  But  for  some  of  the  most  im- 
portant parts  of  the  microscopical  study  of  rocks  a  rotating 
stage  is  requisite,  the  presence  of  which  necessarily  adds  to 
the  cost  or  the  instrument.  One  of  the  best  microscopes 
specially  adapted  for  petrographical  research  is  that  devised 
by  Mr.  A.  Dick,  and  manufactured  by  Swift  &  Son,  of  81 
Tottenham  Court  Road,  London,  price  £18  without  objec- 
tives. 

Among  the  indispensable  adjuncts  are  two  Nicol-prisms, 
one  (polarizer)  to  be  fitted  below  the  stage,  the  other  (ana- 
lyzer) most  advantageously  placed  over  the  eye-piece.  A 
quartz-wedge  is  useful  in  examination  with  polarized  light. 
A  nose-piece  for  two  objectives,  screwed  to  the  foot  of  the 
tube,  saves  time  and  trouble  by  enabling  the  observer  at 
once  to  pass  from  a  low  to  a  high  power.  The  numerous 
pieces  of  apparatus  necessary  for  physiological  work  are  not 
needed  in  the  examination  of  rocks  and  minerals. 

8.  Methods  of  Examination. — A  few  hints  may  be  here  given 
for  the  guidance  of  the  student  in  making  his  own  micro- 
scopic observations,  but  he  must  consult  some  of  the  special 
treatises,  mentioned  on  p.  161,  for  full  details. 

Reflected  Light.  —It  is  not  infrequently  desirable  to  ob- 
serve with  the  microscope  the  characters  of  a  rock  as  an 
opaque  object.  This  cannot  usually  be  done  with  a  broken 
fragment  of  the  stone,  except  of  course  with  very  low  pow- 
ers. Hence  one  of  the  most  useful  preliminary  examinations 
of  a  prepared  slice  is  to  place  it  in  the  field,  and,  throwing 
the  mirror  out  of  gear,  to  converge  as  strong  a  light  upon  it 
as  can  be  had,  short  of  bright  direct  sunlight.  The  observer 
can  then  see  some  way  into  the  rock  and  observe  the  rela- 
tive^ thicknesses  and  forms  of  its  constituents.  The  advan- 
tage' of  this  method  is  particularly  noticeable  in  the  case  of 
opaque  minerals.  The  sulphides  and  iron-oxides  so  abun- 
dant in  rocks  appear  as  densely  black  objects  with  trans- 
mitted light,  and  show  only  their  external  form.  But  by 
throwing  a  strong  light  upon  their  surface,  we  may  often 


GEOGNOSY  169 

discover  not  only  their  distinctive  colors,  but  their  charac- 
teristic internal  structure.  Titaniferous  iron  is  an  admirable 
example  of  the  advantage  of  this  method.  Seen  with  trans- 
mitted light,  that  mineral  appears  in  black,  structureless 
grains  or  opaque  patches,  though  frequently  bounded  by 
definite  lines  and  angles.  But  with  reflected  light,  the 
cleavage  and  lines  of  growth  of  the  mineral  can  then  often 
be  clearly  seen,  and  what  seemed  to  be  uniform  black 
patches  are  found  in  many  cases  to  inclose  bright  brassy 
kernels  of  pyrite.  Magnetite  also  presents  a  characteristic 
blue-black  color,  which  distinguishes  it  from  the  other 
iron-oxides. 

Transmitted  Light. — It  is,  of  course,  with  the  light 
allowed  to  pass  through  prepared  slices  that  most  of  the 
microscopic  examination  of  minerals  and  rocks  is  per- 
formed. A  little  experience  will  show  the  learner  that, 
in  viewing  objects  in  this  way,  he  may  obtain  somewhat 
different  results  from  two  slices  of  the  same  rock  according 
to  their  relative  thinness.  In  the  thicker  one,  a  certain 
mineral  or  rock,  obsidian  for  example,  will  appear  perhaps 
brown  or  almost  black,  while  in  the  other  what  is  evidently 
the  same  substance  may  be  pale  yellow,  green,  brown,  or 
almost  colorless.  Triclinic  felspars  seen  in  polarized  light 
give  only  a  pale  milky  light  when  extremely  thin,  but 
present  bright  chromatic  bands  when  somewhat  thicker. 

Polarized  Light. — By  means  of  polarized  light,  an  ex- 
ceedingly delicate  method  of  investigation  is  made  avail- 
able. We  use  both  the  Nicol-prisms.  If  the  object  be 
singly-refracting,  such  as  a  piece  of  glass,  or  an  amorphous 
body,  or  a  crystal  belonging  to  some  substance  which  crys- 
tallizes in  the  isometric  or  cubic  system  (or  if  it  be  a  tetrag- 
onal, hexagonal  or  rhombohedral  crystal,  cut  perpendicular 
to  its  principal  axis),  the  light  will  reach  our  eye  apparently 
unaffected  by  the  intervention  of  the  object.  The  field  will 
remain  dark  when  the  axes  of  the  two  prisms  are  at  right 
angles  (crossed  Nicols),  in  the  same  way  as  if  no  interven- 
ing object  were  there.  Such  bodies  are  isotropic.™  In  all 
other  cases,  the  substance  is  doubly-refracting  and  modifies 
the  polarized  beam  of  light.  On  rotating  one  of  the  prisms, 
we  perceive  bands  or  flashes  of  color,  and  numerous  lines 
appear  which  before  were  invisible.  The  fie.d  no  longer  re- 
mains dark  when  the  two  Nicol-prisms  are  crossed.  Such 
a  substance  is  anisotropic. 

M  But  the  effect  of  pressure  may  give  weak  color-tints  in  glasses  and  in 
cubic  crystals. 

GEOLOGY— Vol.  XXIX— 8 


170  TEXT-BOOK    OF   GEOLOGY 

It  is  evident,  therefore,  that  we  may  readily  tell  by  this 
means  whether  or  not  a  rock  contains  any  glassy  constit- 
uent. If  it  does,  then  that  portion  of  its  mass  will  become 
dark  when  the  prisms  are  crossed,  while  the  crystalline 
parts  which,  in  the  vast  majority  of  cases,  do  not  belong 
to  the  cubic  system,  will  remain  conspicuous  by  their 
brightness.  A  thin  plate  of  quartz  makes  this  separation 
of  the  glassy  and  crystalline  parts  of  a  rock  even  more 
satisfactory.  It  is  placed  between  the  Nicol-prisms,  which 
may  be  so  adjusted  with  reference  to  it  that  the  field  of  the 
microscope  appears  uniformly  violet.  The  glassy  portion 
of  any  rock,  being  singly-refracting  or  isotropic,  placed  on 
the  stage  will  allow  the  violet  light  to  pass  through  un- 
changed, but  the  crystalline  portions,  being  doubly-refract- 
ing or  anisotropic,  will  alter  trie  violet  light  into  other  pris- 
matic colors.  The  object  should  be  rotated  in  the  field, 
and  the  eye  should  be  kept  steadily  fixed  upon  one  portion 
of  the  slide  at  a  time,  so  that  any  change  may  be  observed. 
This  is  an  extremely  delicate  test  for  the  presence  of  glassy 
and  crystalline  constituents. 

In  searching  for  the  crystallographic  system  to  which 
a  mineral  in  a  microscopic  slide  should  be  referred,  atten- 
tion is  given  to  the  directions  in  which  the  mineral  placed 
between  crossed  Nicols  appears  dark,  or  to  what  are  called 
the  directions  of  its  extinction.  It  is  extinguished  (that  is, 
the  normal  darkness  of  the  field  between  the  crossed  Nicols 
is  ^restored)  when  two  of  its  axes  of  elasticity  for  vibrations 
of  light  coincide  with  the  principal  sections  of  the  two 
prisms.  During  a  complete  rotation  of  the  slide  in  the 
field  of  the  microscope  the  mineral  becomes  dark  in  four 
positions  90°  apart,  each  of  which  marks  that  coincidence. 
W  hen,  on  the  other  hand,  the  prisms  are  placed  parallel  to 
each  other,  the  coincidence  of  their  principal  sections  with 
the  axes  of  elasticity  in  the  mineral  allows  the  maximum 
of  light  to  pass  through,  which  likewise  occurs  four  times 
in  a  complete  rotation  of  the  mineral.  The  different  crys- 
tallographic systems  are  distinguishable  by  the  relation  be- 
tween their  crystallographic  axes  and  their  axes  of  elasticity. 
By  noting  this  relation  in  the  case  of  any  given  mineral  (and 
there  are  usually  sections  enough  of  each  mineral  in  the 
same  rock-slice  to  furnish  the  required  data)  its  crystalline 
system  may  be  fixed.  But  in  many  cases  it  has-been  found 
possible  to  establish  characteristic  distinctions  for  individual 
mineral  species,  by  noting  the  angle  between  the  direction 
of  their  extinction  and  certain  principal  faces. 


GEOGNOSY  171 

The  determination  of  whether  the  component  grains  of 
a  rock  belong  to  uniaxial  or  biaxial  doubly-refracting  min- 
erals is  a  point  of  much  importance,  which  is  effected  by 
means  of  an  achromatic  condenser  inserted  in  the  aperture 
of  the  stage  below  the  slide  and  suitably  adjusted  so  as  to 
converge  the  rays  of  light  within  the  grain  or  crystal.  The 
Nicols  having  been  crossed,  the  eye-piece  is  removed,  and 
the  eye  when  held  a  little  distance  from  the  open  end  of  the 
tube  will  perceive  a  dark  bar,  ring,  or  cross  move  across 
the  field  as  the  stage  is  rotated,  if  the  mineral  examined 
has  been  cut  at  a  Favorable  angle.  By  the  form  and  be- 
havior of  these  indications  the  uniaxial  or  biaxial  character 
is  made  evident. 

Pleochroism  (Dichroism). — Some  minerals  show  a  change 
of  color  when  a  Nicol-pnsm  is  rotated  below  them;  horn- 
blende, for  example,  exhibiting  a  gradation  from  deep 
brown  to  dark  yellow.  A  mineral  presenting  this  change 
is  said  to  be  pleochroic  (polychroic,  dichroic,  trichroic). 
To  ascertain  the  pleochroism  of  any  mineral  we  may  re- 
move the  upper  polarizing  prism  (analyzer)  and  leave  only 
the  lower  (polarizer).  If  as  we  rotate  the  latter,  no  change 
of  tint  can  be  observed,  there  is  no  pleochroic  mineral 
present,  or  at  least  none  which  shows  pleochroism  at  the 
angle  at  which  it  has  been  bisected  in  the  slice.  But  in 
a  slice  of  any  crystalline  rock,  crystals  may  usually  be 
observed  which  offer  a  change  of  hue  as  the  prism  goes 
round.  These  are  examples  of  pleochroism.  This  behavior 
may  be  used  to  detect  the  mineral  constituents  of  rocks. 
Thus  the  two  minerals  hornblende  and  augite,  which  in  so 
many  respects  resemble  each  other,  cannot  always  be  distin- 
guished by  cleavage  angles,  in  microscopic  slices.  But  as 
Tschermak  pointed  out,  augite  remains  passive  or  nearly 
so  as  the  lower  prism  is  rotated:  it  is  not  pleochroic,  or 
only  very  feebly  so;  while  hornblende,  on  the  other  hand, 
especially  in  its  darker  varieties,  is  usually  strongly  pleo-v 
chroic.  It  is  to  be  observed,  however,  that  the  same  mineral 
is  not  always  equally  pleochroic,  and  that  the  absence  of 
this  property  is  therefore  less  reliable  as  a  negative  test, 
than  its  presence  is  as  a  positive  test. 

It  would  be  beyond  the  scope  of  this  volume  to  enter 
into  the  complicated  details  of  the  microscopic  structure 
of  minerals  and  rocks.  This  information  must  be  sought 
in  some  of  the  works  specially  devoted  to  it,  a  few  of  which 
are  cited  on  p.  161. 

In  his  examination  of  rocks  with  the  microscope,  the 


172  TEXT-BOOK   OF   GEOLOGY 

student  may  find  an  advantage  in  propounding  to  himself 
the  following  questions,  and  referring  to  the  pages  here 
cited. 

1st,  Is  the  rock  entirely  crystalline  (pp.  174,  258,  268), 
consisting  solely  of  crystals  of  different  minerals  interlaced; 
and  if  so,  what  are  these  minerals  ?  2d,  Is  there  any  trace 
of  a  glassy  ground-mass  or  base  (pp.  178,  204)?  Should 
this  be  detected,  the  rock  is  certainly  of  volcanic  origin 
(pp.  282,  297).  3d,  Can  any  evidence  be  found  of  the  de- 
vitrification of  what  may  have  been  at  one  time  the  glassy 
basis  of  the  whole  rock?  This  devitrification  might  be 
shown  by  the  appearance  of  numerous  microscopic  hairs, 
rods,  bundles  of  feather-like  irregular  or  granular  aggrega- 
tions (p.  204).  4th,  In  what  order  did  the  minerals  crystal- 
lize? This  may  often  be  made  out  with  a  microscope,  as, 
for  instance,  where  one  mineral  is  inclosed  within  another 
(p.  204)."  5th,  "What  is  the  nature  of  any  alteration  which 
the  rock  may  have  undergone  ?  In  a  vast  number  of  cases 
the  slices  show  abundant  evidence  of  such  alteration:  fel- 
spar passing  into  granular  kaolin,  augite  changing  into 
viridite,  olivine  into  serpentine,  while  secondary  calcite, 
epidote,  quartz,  and  zeolites  run  in  minute  veins  or  fill  up 
interstices  of  the  rock  (p.  587).  6th,  Is  the  rock  a  frag- 
mental  one;  and  if  so,  what  is  the  nature  of  its  component 
grains  (pp.  224-225)?  Is  any  trace  of  organic  remains  to 
be  detected? 

§  iv.—  General  outward  or  Megascopic  (Macroscopic)  Characters 
of  Rocks68 

1.  Structure.69 — The  different  kinds  of  rock-structures  dis- 


51  It  is  possible,  however,  that  a  crystal  inclosed  within  another  may  some- 
times have  crystallized  there  out  of  a  portion  of  the  surrounding  magma  of  the 
rock  whicli  has  been  inclosed  within  the  larger  crystal  (postea  p.  514). 

68  The  following  general  text-books  on  rocks  may  be  referred  to:  Maccul- 
loch,  "A  Geological  Classification  of  Rocks,"  etc.,  London,  1821.  B.  von 
Cotta,  "Rocks  Classified  and  Described,"  translated  by  Lawrence,  London, 
1866.  Zirkel,  "Lehrbuch  der  Petrographie, "  two  vols.  Bonn,  1866.  Senft, 
"Classification  der  Felsarten,"  Breslau,  1857;  "Die  Krystallinischen  Felsge- 
mengtheile,"  Berlin,  1868.  Ken ngott,  "Elemente  der  Petrographie,"  Leipz., 
1868.  A.  von  Lasaulx,  "Elemente  der  Petrographie,"  Bonn,  1876.  Bischof, 
"Chemical  Geology,"  translated  for  Cavendish  Society,  1854-59,  and  supple- 
ment, Bonn,  1871.  Roth,  "Allgemeine  und  Chemische  Geologic,"  Berlin, 
1879.  Other  works  in  which  the  microscopical  characters  are  more  specially 
treated  of,  are  enumerated  on  p.  193. 

59  In  the  3d  edition  of  Jukes'  "Student's  Manual  of  Geology"  (1871),  p.  93, 
it  was  proposed  to  reserve  the  term  "Structure"  for  large  features,  such  as  char- 
acterize rock-blocks,  and  to  use  the  term  "Texture"  for  the  minuter  characters, 


GEOGNOSY  173 

tinguishable  by  the  unaided  eye  are  denoted  either  by  ordi- 
nary descriptive  adjectives,  or  by  terms  derived  from  rocks 
in  which  the  special  structures  are  characteristically  de- 
veloped, such  as  granitoid,  brecciated,  shaly.  It  must  be 
borne  in  mind,  bowever,  that  the  external  character  of 
a  rock  does  not  always  supply  us  with  its  true  internal 
structure,  which  may  be  gained  only  by  microscopic  ex- 
amination. This  is  of  course  more  especially  true  of  the 
close-grained  kinds,  where  to  the  naked  eye  no  definite 
structure  is  discernible.  Some  of  the  definitions  originally 
founded  on  external  appearance  have  been  considerably 
modified  by  microscopic  investigation.  Many  compact 
rocks,  for  instance,  have  been  proved  to  be  wholly  crys- 
talline. 

The  same  rock- mass  may  show  very  different  structures 
and  textures  in  different  parts  of  its  extent.  This  is  true 
alike  of  sedimentary  and  igneous  materials.  It  may  be 
observed  even  in  the  several  portions  of  one  continuous 
mass  of  erupted  rock — variations  in  the  rate  of  cooling,  in 
temperature,  and  other  circumstances  have  combined  to 
produce  sometimes  the  most  extraordinary  textural  and 
even  structural,  as  well  as  chemical  and  mineralogical 
contrasts  in  a  boss  or  sheet  of  igneous  rock."  Hence  the 
student  must  be  on  his  guard  against  concluding  that  two 
portions  of  rock  strikingly  unlike  each  other  in  outward 
appearance  cannot  be  portions  of  one  original  continuous 
mass. 

such  as  can  be  judged  of  in  hand  specimens.  M.  De  Lapparent  makes  a  similar 
distinction  (Traite",  p.  602,  note).  But  the  practice  of  using  the  word  structure 
as  it  is  employed  above  in  the  text,  has  received  such  a  support  from  the  petrog- 
raphers  of  Germany  that  though  I  still  think  it  would  be  preferable  to  distin- 
guish between  texture  and  structure,  I  have  adopted  what  has  now  the  sanction 
of  common  usage. 

60  See  Book  IV.  Part  VTI.  ;  G.  P.  Becker,  Amer.  Journ.  Sci.  xxxiii.  (1887), 
p.  50.  J.  H.  L.  Vogt,  GeoL  Foren.  Forhand.  Stockholm,  xiii.  (1891). 


174  TEXT-BOOK   OF   GEOLOGY 

Crystalline  (P  h  a  n  er  oc  ry  sta  1 1  i  n  e),  consisting 
wholly  or  chiefly  of  crystalline  particles  or  crystals.61 
Where  the  individual  elements  of  the  rocks  are  of  large 
size,  the  structure  is  coarse-crystalline  (granitic),  as  in  many 
granites.  When  the  particles  are  readily  visible  to  the 
naked  eye,  and  are  tolerably  uniform  in  size,  as  in  marble, 
many  granites  and  dolomites,  the  rock  is  said  to  be  granular- 
crystalline.  Successive  stages  in  the  diminution  of  the  size 
of  the  particles  may  be  traced  until  these  are  no  longer  rec- 
ognizable with  the  naked  eye,  and  the  structure  must  then 
be  resolved  with  the  microscope  (fine-crystalline,  micro-crys- 
talline, crypto-crystalline).  Fine-grained  rocks  may  also  be 
called  compact,  though  this  term  is  likewise  applicable  to 
the  more  close-grained  varieties  of  the  fragmental  series. 
The  microscopic  characters  of  such  rocks  should  always 
be  ascertained  where  possible." 

Many  crystalline  rocks  consist  not  only  of  crystals,  but 
of  a  magma  or  paste,  in  which  the  crystalline  particles  are 
seen  by  the  naked  eye  to  be  imbedded.  It  is  of  course  im- 
possible, except  from  analogy,  to  determine  macroscopically 
what  may  be.  the  nature  of  this  magma.  It  may  be  entirely 
composed  of  minute  crystals,  or  may  consist  of  various 
crystallitic  products  of  devitrification.  Its  intimate  struc- 
ture can  only  be  ascertained  with  the  microscope.  But  its 
existence  is  often  strikingly  manifest  even  to  the  unassisted 
eye,  for  in  what  are  termed  "porphyries"  it  forms  a  large 
part  of  their  mass.  The  term  "ground-mass"  is  employed 
to  denote  this  megascopic  matrix.  Microscopic  examination 
shows  that  a  ground-mass  may  consist  of  minute  crystals, 

61  Prof.  Rosenbusch  proposed  the  term  "holocrystalline"  for  rocks  in  which 
there  is  no  morphous  material  among  the  crystalline  constituents. 

61  On  the  crystallization  of  igneous  rocks,  see  J.  P.  Iddings,  Bull.  Phil.  Soc. 
Washington,  xi.  (1889),  p.  71. 


GEOGNOSY  175 

or  crystallites,  or  granules  and  filaments,  or  glass,  or  combi- 
nations of  these  in  various  proportions.     (See  pp.  194,  207.) 

Lithoid,  compact  and  stony  in  aspect,  with  no  very 
distinct  crystalline  structure.  The  term  is  especially  ap- 
plied to  the  devitrified  condition  of  once  glassy  rocks, 
such  as  obsidians,  which  have  assumed  the  character  of 
perlites  or  f  el  sites. 

Granitic  (Granitoid),  thoroughly  crystalline,  and 
consisting  of  crystals  approximately  uniform  in  size,  as  in 
granite.  This  structure  is  characteristic  of  many  eruptive 
rocks.  Though  usually  distinctly  recognizable  by  the 
naked  eye  ("macromerite"  of  Vogelsang83),  it  sometimes 
becomes  very  fine  ("micromerite"),  and  may  be  only 
recognizable  with  the  microscope  as  thoroughly  crystal- 
line (microgranitic);  at  other  times  it  passes  into  a  porphy- 
ritic  or  porphyroid  character  by  the  appearance  of  large 
crystals  dispersed  through  a  'general  ground-mass. 

Pegmatitic  (Pegmatoid,  Graphic),  exhibiting 
the  peculiar  arrangement  of  crystalline  constituents  seen  in 
pegmatite  or  graphic  granite  (p. 
275),  where  the  quartz  and  felspar 
have  crystallized  simultaneously 
so  as  to  be  inclosed  within  each 
other.  This  structure  may  be 
seen  on  a  large  scale  in  many 
massive  veins  of  pegmatite;  where 
it  takes  an  exceedingly  minute 
form  it  is  known  as  micropeg- 

.      .  Fig.  5.— Micropesrmatitic  Structure, 

m  a  1 1 1 1  C    (r  Ig.    5).       Such    micro-     wanophyre,  Mull.    (Magnified.) 

scopic  intergrowth  of  quartz  and  felspar  is  characteristic  of 
large  masses  of  eruptive  rock  (micropegmatite,  granophyre). 

68  Z.  Deutsch.  Geol.  Ges.  xxiv.  p.  534. 


176  TEXT-BOOK    OF   GEOLOGY 

Aphanitic,  a  name  given  to  the  very  close  texture 
exhibited  by  some  igneous  rocks  (diabases,  diorites)  where 
the  component  ingredients  cannot  be  determined  except  with 
the  microscope. 

Porphyritic  (Porphyroid),  composed  of  a  com- 
pact or  finely  crystalline  ground-mass,  through  which  larger 
crystals  of  earlier  consolidation,64  often  of  felspar,  are  dis- 
persed (Fig.  6).  This  and  the  granitic  structure  are  the  two 


Fig.  6. — Porphyritic  Structure.    (Nat.  size.) 

great  structure- types  of  the  eruptive  rocks.  By  far  the 
larger  number  of  these  rocks  belong  to  the  porphyritic  type. 
Microscopic  research  has  thrown  much  light  on  the  nature 
of  the  ground-mass  of  porphyritic  rocks.  Vogelsang  pro- 
posed to  classify  these  rocks  in  three  divisions:  "  1st,  Grano- 
phyre,  where  the  ground-mass  is  a  microscopic  crystalline 
mixture  of  the  component  minerals  with  absence  or  sparing 
development  of  an  imperfectly  individualized  magma  (see 
p.  209);  2d,  Felsophyre,  having  usually  an  imperfectly  indi- 
vidualized or  felsitic  magma  for  the  ground-mass  (pp.  208, 


64  Phenocrysts,  Iddings,  Bull.  Phil.  Soc.  Washington,  ii.  (1889),  p.  13. 

65  Vogelsang,  loc.  cit.     Compare  the  classification  into  granitoid  and  trachy- 
toid,  p.  271. 


GEOGNOSY  177 

211);  3d,  Vitrophyre,  where  the  ground-mass  is  a  glassy 
magma  (pp.  204,  212).  The  second  subdivision  embraces 
most  of  the  porphyries,  and  a  very  large  number  of  eruptive 
rocks  of  all  ages.66 

Segregated. — In  granite  and  other  crystalline  mas- 
sive rocks,  vein-like  portions,  coarser  (or  finer)  in  texture 
than  the  rest  of  the  mass,  may  be  observed.  .  These  belong 
to  the  last  phase  of  consolidation,  when  segregations  from 
the  original  molten  or  viscous  magma  took  place  along  cer- 
tain lines  or  round  particular  centres,  where  the  individual 
minerals  crystallized  out  from  the  general  mass.  They  have 
been  sometimes  termed  "segregation,"  or  "exudation" 
veins.  They  are  to  be  distinguished  from  the  veins, 
usually  of  finer  and  more  acid  material,  which  ramify 
through  a  mass  of  igneous  rock  and  probably  represent 
portions  of  the  original  molten  magma  which  remained 
still  liquid  and  were  injected  into  rents  of  the  already  con- 
solidated parts.  These  are  the  true  "contemporaneous 
veins"  (Book  IV.  Part  VII.) 

Gr  r  a  n  u  1  a  r — a  somewhat  vague  term  applied  to  rocks 
composed"  of  approximately  equal  grains,  which  are  some- 
times worn  fragments,  as  in  sandstone,  sometimes  crystalline 
particles,  as  in  granite  and  marble.  This  texture  may  be- 
come so  fine  as  to  pass  insensibly  into  compact.'7  The 
peculiar  granular  structure  found  so  abundantly  among 
metamorphic  rocks  wkich  have  been  intensely  crushed  and 
in  which  there  seems  to  have  been  a  process  of  recrystalli- 


66  According  to  Rosenbusch  the  porphyritic  massive  rocks  are  those  in 
which,  during  the  different  stages  of  their  produciion,  the  same  minerals 
have  been  formed  more  than  once.  Neuea  Jahrb.  1882  (ii.),  p.  14. 

61  As  applied  to  massive  (eruptive)  rocks,  Rosenbusch  would  restrict  the 
term  granular  to  those  in  which  each  individual  constituent  separated  out  during 
but  one  definite  stage  of  the  process  of  rock-building.  Loc.  cit. 


178  TEXT-BOOK    OF   GEOLOGY 

zation  among  the  powdered  particles,  has  been  termed 
gra  null  tic  (p.  210).  This  word,  however,  is  liable  to 
the  objection  that  in  Germany  it  is  applied  to  rocks  bearing 
that  structure  while  in  France  it  is  used  for  a  holocrystalline 
granite.68 

Vitreous  or  glassy,  having  a  structure  like  that  of 
artificial  glass;  as  in  obsidian.  Among  the  crystalline  rocks 
there  is  often  present  a  variable  amount  of  an  amorphous 
ground-mass,  which  may  increase  until  it  forms  the  main 
part  of  the  substance.  The  nature  of  this  amorphous  por- 
tion is  described  at  pp.  203,  212.  Its  most  obvious  mega- 
scopic condition  is  that  of  a  volcanic  glass.  Most  vitreous 
rocks  present,  even  to  the  naked  eye,  dispersed  grains,  crys- 
tals, or  other  inclosures.  Under  the  microscope,  they  are 
found  to  be  often  crowded  with  minute  crystals  and  imper- 
fect or  incipient  crystalline  forms  (pp.  194,  205).  Resin- 
ous  is  the  term  applied  to  vitreous  rocks  having  the  lustre 
of  pitchstone,  and  to  others  which  are  still  less  vitreous. 
Devitrification  is  the  conversion  of  the  vitreous  into  a  crys- 
talline or  lithoid  structure  (pp.  206,  214). 

Streaked,  arranged  in  streaky  inconstant  lines  (Germ. 
Schliereri),  either  parallel  or  convergent,  and  often  undulat- 
ing. This  structure,  conspicuously  shown  by  the  lines  of 
flow  in  vitreous  rocks  (flow-structure,  fluxion-structure, 
fluidal-structure)  is  less  marked  where  the  materials  have 
assumed  definite  crystalline  forms.  It  can  be  seen  on  a 
minute  scale,  however,  in  many  crystalline  masses  when 
examined  with  the  microscope  (p.  211). 

Banded,  arranged  in  parallel  bands,  distinguished  from 
each  other  by  color,  texture,  structure  or  composition ;  char- 

68  Michel-Levy,  Ann.  des  Mines,  viii.  (1875),  p.  387;  "Structure  et  Classifi- 
cation des  Roches  Eruptives,"  1889,  p.  14. 


GEOGNOSY  179 

acteristic  of  many  gneisses,  and  of  jaspers,  flints,  halleflintas 
and  other  flinty  rocks.  This  term  may  frequently  be  ap- 
plied to  the  flow-structure  of  igneous  rocks  referred  to  in 
the  previous  paragraph,  likewise  to  the  segregation  veins  of 
eruptive  bosses  and  sheets,  and  to  the  parallel  arrangement 
of  materials  produced  in  rocks  which  have  under  intense 
mechanical  pressure  been  crushed  and  sheared.  With  the 
naked  eye  it  is  often  hardly  possible  to  distinguish  between 
the  banded  structure  of  devitrified  igneous  rocks  and  that 
resulting  from  intense  mechanical  deformation. 

Mylonitic,  a  term  introduced  to  denote  the  peculiar 
granular  structure  of  rocks  which  have  undergone  intense 
crushing.  The  materials  have  been  reduced  to  minute  grains 
which  have  not  recrystallized  as  they  have  done  in  the  gran- 
ulitic  structure.  Many  remarkable  examples  of  this  struc- 
ture have  been  observed  among  the  schists  of  the  Scottish 
Highlands. 

Spherulitic,  composed  of,  or  containing  small  glob- 
ules or  spherules  which  may 
be  colloid  and  isotropic,  or 
more  or  less  distinctly  crystal- 
line, particularly  with  an  internal 
fibrous  divergent  structure  (Figs. 
7,  17).  This  structure  occurs  in 
vitreous  rocks,  where  it  is  one  of 
the  stages  of  devitrification  in 

Obsidian,  pitchstone,  etc.'9  (p.  214).        Fig. T.-SpherulRU^Structure. 

69  On  the  constitution  and  origin  of  spherulite  in  acid  eruptive  rocks,  see 
"Whitman.  Cross,  Phil.  Soc.  "Washington,  xi.  p.  411  (1891),  and  J.  P.  Iddings, 
op.  cit.  p.  445.  Quartz  assumes  in  some  rocks  (e.g.  banded  eurites)  a  finely 
globular  structure  which  was  developed  before  the  cessation  of  the  motion  that 
produced  flow -structure,  and  which,  according  to  M.  Michel-Levy,  may  be  re- 
garded as  connecting  the  colloid  and  crystallized  conditions  of  silica.  Bull.  Soc. 
Geol.  France  (3),  v.  p.  140. 


180  TEXT-BOOK   OF   GEOLOGY 

The  term  lithophyse  has  been  applied  by  F.  von 
Richthofen  to  large  bladder-like  sphemlites  wherein  inter- 
spaces lined  with  crystals  occur  between  the  successive  con- 
centric internal  layers.70  Many  ancient  rhyolites  present  an 
aggregate  of  nodular  bodies  (Pyromeride)  due  originally  to 
devitrification  and  subsequently  more  or  less  altered  espe- 
cially by  the  deposition  of  silica  within  them  (postea,  p.  260). 

Orbicular  structure  is  one  in  which  the  component 
minerals  of  a  rock  have  crystallized  in  such  a  way  as  to 


Fig.  8.— Orbicular  Structure.    Napoleonite,  Corsica.    (Nat.  Size.) 

form  spheroidal  aggregations  sometimes  with  an  internal 
radial  or  concentric  grouping.  It  is  typically  seen  in  the 
napoleonite  or  ball-diorite  (kugeldiorite,  orbicular  diorite, 
p.  287)  of  Corsica  (Fig.  8),  but  occurs  in  other  rocks,  some- 
times even  in  granite. 

P  e  r  1  i  t  i  c  (Figs.  9  and  20),  having  the  structure  of  the 
rock  formerly  termed  perlite,  wherein  between  minute  rec- 

70  Jahrb.  K.  K.  Geol.  Reichsanst,  1860,  p.  180.     See  Iddings,  7th  Ann.  Rep. 
U.  S.  Geol.  Surv.  (1885-86),  p.  249.     Amer.  Journ.  Sci.  xxxiii.  (1887),  p.  36. 


GEOGNOSY 


181 


Pig.  9  — Perlitic  Structure. 

(Magnified.) 


tilinear  fissures  the  substance  of  the  mass  has  assumed, 
during  the  contraction  resulting  from  cooling,  a  finely 
globular  character,  not  unlike  the 
spheroidal  structure  seen  in 
weathered  basalt  which  is  also  a 
phenomenon  of  contraction  dur- 
ing the  cooling  and  consolidation 
of  an  igneous  rock. 

Horny,  flinty,  having  a 
compact,  homogeneous,  dull  text- 
ure, like  that  of  horn  or  flint,  as 
in  chalcedony,  jasper,  flint,  and 
many  halleflintas  and  felsites. 

Cavernous  (porous),  containing  irregular  cavities 
due,  in  most  cases,  to  the  abstraction  of  some  of  the  min- 
erals; but  occasionally,  as  in  some  limestones  (sinters), 
dolomites  and  lavas,  forming  part  of  the  original  struc- 
ture of  the  rock. 

C  e  1 1  u  1  a  r. — Many  lavas,  ancient  and  modern,  have 
been  saturated  with  steam  at  the  time  of  their  eruption, 
and  in  consequence  of  the  segregation  and  expansion  of 
this  imprisoned  vapor,  have  had  spherical  cavities  devel- 
oped in  their  mass.  When  this  cellular  structure  is  mark'ed 
by  comparatively  few  and  small  holes,  it  may  be  called 
vesicular;  where  the  rock  consists  partly  of  a  roughly 
cellular,  and  partly  of  a  more  compact  substance  inter- 
mingled, as  in  the  slag  of  an  iron  furnace,  it  is  said  to  be 
slaggy;  portions  where  the  cells  occupy  about  as  much 
space  as  the  solid  part,  and  vary  much  in  size  and  shape, 
are  called  scoriaceous,  this  being  the  character  of  the 
rough  clinker-like  scoriae  of  recent  lava-streams;  when  the 
cells  are  so  much  more  numerous  than  the  solid  part,  that 


182  TEXT-BOOK   OF   GEOLOGY 

the  stone  would  almost  or  quite  float  on  water,  the  structure 
is  called  pumiceous,  the  term  pumice  being  applied  to 
the  froth-like  part  of  obsidian.  As  the  cellular  structure 
can  only  be  developed  while  the  rock  is  still  liquid,  or  at 
least  viscid,  and  as,  while  in  this  condition,  the  mass  is 
often  still  moving  away  from  its  point  of  emission,  the 
cells  are  not  infrequently  elongated  in  the  direction  of 
movement.  Subsequently  water,  infiltrating  through  the 


Fig.  10.— Amygdaloidal  Structures;  Porphyrite,  Old  Red  Sandstone,  Ayrshire. 
(Nat.  size.) 

rock,  deposits  various  mineral  substances  (calcite,  quartz, 
chalcedony,  zeolites,  etc.)  from  solution,  so  that  the  flat- 
tened and  elongated  almond-shaped  cells  are  eventually 
filled  up.  A  cellular  rock  which  has  undergone  this  change 
is  said  to  be  an  amygdaloid,  or  arnygdaloidal,  and 
the  almond-like  kernels  are  known  as  amygdales  (Fig. 
10).  Where  the  cells  or  cavernous  spaces  of  a  rock  are 
lined  with  crystals  and  empty  inside  they  are  said  to  be 
druses  or  drusy  cavities. 

Cleaved,  having  a  fissile  structure  superinduced  by 
pressure  and  known  as  cleavage  (see  pp.  531,  532).  The 
planes  of  cleavage  are  independent  of  those  of  bedding, 


GEOGNOSY  183 

though  they  may  coincide  with  them.  A  cleaved  structure 
is  best  seen  in  fine-grained  material,  and  is  typically  devel- 
oped in  roofing-slate,  but  it  may  occur  in  any  compact 
igneous  rock. 

Foliated,  consisting  of  minerals  that  have  crystallized 
in  approximately  parallel,  lenticular,  and  usually  wavy 
layers  or  folia.  Rocks  of  this  kind  commonly  contain 
layers  of  mica,  or  of  some  equivalent  readily  cleavable 
mineral,  the  cleavage-planes  of  which  coincide  generally 
with  the  planes  of  foliation.  Gneiss,  mica-schist  and  talc- 
schist  are  characteristic  examples.  So  distinctive,  indeed, 
is  this  structure  in  schists,  that  it  is  often  spoken  of  as 
schistose.  In  gneiss,  it  attains  its  most  massive  form; 
in  chlorite-schist  and  some  other  schists,  it  becomes  so 
fine  as  to  pass  into  a  kind  of  minutely  scaly  texture,  often 
only  perceptible  with  the  microscope,  the  rock  having  on 
the  whole  a  massive  structure. 

Fibrous,  consisting  of  one  or  more  minerals  composed 
of  distinct  fibres.  Sometimes  the  fibres  are  remarkably 
regular  and  parallel,  as  in  fibrous  gypsum,  and  veins  of 
chrysotile,  fibrous  aragonite  or  calcite  (satin-spar);  in  other 
instances,  they  are  more  tufted  and  irregular,  as  in  asbestos 
and  actinolite-schist. 

Clastic,  fragmental,  composed  of  detritus  (p.  214). 
Kocks  possessing  this  character  have,  in  the  great  majority 
of  cases,  been  formed  in  water,  and  their  component  frag- 
ments are  usually  more  or  less  rounded  or  water-worn. 
Different  names  are  applied,  according  to  the  form  or 
size  of  the  fragments.  Brecciated,  composed,  like  a 
breccia,  of  angular  fragments,  which  may  be  of  any  degree 
of  coarseness.  Agglomerated,  consisting  of  large, 
roughly  rounded  and  tumultuously  grouped  blocks,  as 


184  TEXT-BOOK   OF   GEOLOGY 

in  the  agglomerate  filling  old  volcanic  funnels.  Con- 
glomerated (Conglomeratic),  made  up  of  well- 
rounded  blocks  or  pebbles;  rocks  having  this  character 
have  been  formed  by  and  deposited  in  water.  Pebbly, 
containing  dispersed  water- worn  pebbles,  as  in  many  coarse 
sandstones,  which  thus  by  degrees  pass  into  conglomerates. 
P  s  a  m  m  i  t  i  c,  or  sandstone-like,  composed  of  rounded 
grains,  as  in  ordinary  sandstone:  when  the  grains  are 
larger  (often  sharp  and  somewhat  angular)  the  rock  is 
gritty,  or  a  grit.  M  uddy  (peli tic),  having  a  tex- 
ture like  that  of  dried  mud.  Cryptoclastic  or  com- 
pact, where  the  grains  are  too  minute  to  reveal  to  the 
naked  eye  the  truly  fragmental  character  of  the  rock,  as 
in  fine  mudstones  and  other  argillaceous  deposits. 

Concretionary,  containing,  or  consisting  of  mineral 
matter,  which  has  been  collected,. either  from  the  surround- 
ing rock  or  from  without,  round  some  centre,  so  as  to  form 
a  nodule  or  irregularly  shaped  lump.  This  aggregation  of 
material  is  of  frequent  occurrence  among  water-formed 
rocks,  where  it  may  be  often  observed  to  have  taken  place 
round  some  organic  centre,  such  as  a  leaf,  cone,  shell,  fish- 
bone, or  other  relic  of  plant  or  animal.  (Book  IV.  Part  I.) 
Among  the  most  frequent  minerals  found  in  concretionary 
forms  as  constituents  of  rocks,  are  calcite,  siderite,  pyrite, 
marcasite,  and  various  forms  of  silica.  In  a  true  concre- 
tion, the  material  at  the  centre  has  been  deposited  first, 
and  has  increased  by  additions  from  without,  either  during 
the  formation  of  the  inclosing  rock,  or  by  subsequent  con- 
centration and  aggregation.  Where,  on  the  other  hand, 
cavities  and  fissures  have  been  filled  up  by  the  deposition 
of  materials  on  their  walls,  and  gradual  growth  inward, 
the  result  is  known  as  a  secretion.  Amygdales  and 


GEOGNOSY  185 

the  successive  coatings  of  mineral  veins  are  examples  of 
the  latter  process. 

Septaria n — a  structure  often  exhibited  by  concretions 
of  limestone  and  clay-ironstone  which  in  consolidating  have 
shrunk  and  cracked  internally.  These  shrinkage-cracks  ra- 
diate in  an  irregular  way  from  the  middle  toward  the  cir- 
cumference, but  die  out  before  reaching  the  latter  (Fig.  26). 
Usually  they  have  been  filled  with  some  subsequently  in- 
filtrated mineral,  notably  calcite. 

Oolitic,  a  structure  like  fish-roe,  formed  of  spherical 
grains,  each  of  which  has  an  internal  radiating  and  concen- 
tric structure,  and  often  possesses  a  central  nucleus  of  some 
foreign  body.  This  structure  is  specially  found  among  lime- 
stones (see  p.  262).  When  the  grains  are  as  large  as  peas, 
the  structure  is  termed  pisolitic. 

Various  structures  which  affect  large  masses  of  rock 
rather  than  hand-specimens  will  be  found  described  in 
Book  IV.  But  a  few  of  the  more  important  may  be 
included  here. 

Massive,  unstratified,  having  no  arrangement  in 
definite  layers  or  strata.  Lava,  granite,  and  generally  all 
crystalline  rocks  which  have  been  erupted  to  the  surface, 
or  have  solidified  below  from  a  state  of  fusion  are  massive 
rocks. 

Stratified,  bedded,  composed  of  layers  or  beds  ly- 
ing parallel  to  each  other,  as  in  shale,  sandstone,  limestone, 
and  other  rocks  which  have  been  deposited  in  water.  Suc- 
cessive streams  of  lava,  poured  one  upon  another,  have  also 
a  bedded  arrangement.  Laminated,  consisting  of  fine, 
leaf-like  strata  or  laminse;  this  structure  being  characteris- 
tically exhibited  in  shales,  is  sometimes  also  called  s  h  a  1  y. 


186  TEXT-BOOK    OF   GEOLOGY 

Jointed,  traversed  by  the  divisional  planes  termed 
Joints  which  are  fully  treated  of  in  Book  IV.  Part  II. 

Columnar,  divided  into  prismatic  joints  or  columns. 
This  structure  is  typically  represented  among  the  basalts 
and  other  basic  lavas  (p.  883  and  Figs.  230-232),  but  it  may 
also  be  observed  as  an  effect  of  contact-metamorphism 
among  stratified  rocks  which  have  been  invaded  by  intru- 
sive masses  (Book^IV.  Part  VIII.) 

2.  Composition* — Before  having  recourse  to  chemical  or 
microscopic  analysis,  the  geologist  can  often  pronounce  as 
to  the  general  chemical  or  mineralogical  nature  of  a  rock. 
Most  of  the  terms  which  he  employs  to  express  his  opinion 
are  derived  from  the  names  of  minerals,  and  in  almost  all 
cases  are  self-explanatory.  The  following  examples  may 
suffice.  Calcareous,  consisting  of  or  containing  car- 
bonate of  lime.  Argillaceous,  consisting  of  or  con- 
taining clay.  Felspathic,  having  some  form  of  felspar 
as  a  main  constituent.  Siliceous,  formed  of  or  contain- 
ing silica;  usually  applied  to  the  chalcedonic  forms  of  this 
cementing  oxide.  Quartzose,  containing  or  consisting 
entirely  of  some  form  of  quartz.  Carbonaceous,  con- 
taining coaly  matter,  and  hence  usually  associated  with 
a  dark  color.  P  y  r i  t o  u  s,  containing  diffused  disulphide 
of  iron.  Grypseous,  containing  layers,  nodules,  strings 
or  crystals  of  calcium-sulphate.  Saliferous,  containing 
beds  of,  or  impregnated  with  rock-salt.  Micaceous,  full 
of  layers  of  mica-flakes. 

As  rocks  are  not  definite  chemical  compounds,  but  mix- 
tures of  different  minerals  in  varying  proportions,  they  ex- 
hibit many  intermediate  varieties.  Transitions  of  this  kind 
are  denoted  by  such  phrases  as  "granitic  gneiss,"  that  is, 
a  gneiss  in  which  the  normal  foliated  structure  is  nearly 


GEOGNOSY  187 

merged  into  the  massive  structure  of  granite;  "argillaceous 
limestone" — a  rock  in  which  the  limestone  is  mixed  with 
clay;  "calcareous  shale" — a  fissile  rock,  consisting  of  clay 
with  a  proportion  of  lime.  It  is  evident  that  such  rocks 
may  graduate  so  insensibly  into  each  other,  that  no  sharp 
line  can  be  drawn  between  them  either  in  the  field  or  in 
their  terminology. 

As  already  alluded  to,  and  as  will  be  more  fully  ex- 
plained in  later  pages,  the  progress  of  research  goes  to  show 
that  even  in  the  same  mass  of  eruptive  rock  considerable 
differences  of  chemical  composition  may  be  found.  These 
differences  seem  to  point  to  some  separation  of  the  constit- 
uents, by  gravity  or  otherwise,  before  consolidation.  Thus 
the  picrite  of  Bathgate  shades  upward  into  a  rock  in  which 
the  heavy  magnesian  silicates  are  replaced  in  large  measure 
by  felspars/1  Mr.  Iddings  has  recently  called  attention  to 
some  remarkable  gradations  of  composition  among  the 
volcanic  rocks  of  the  Tewar  Mountains,  New  Mexico, 
where  be  believes  a  series  of  intermediate  varieties  to  be 
traceable  from  obsidian  at  the  one  end  to  basalt  at  the 
other."  A  remarkable  instance  of  a  similar  kind  is  de- 
scribed by  Mr.  Teall  and  Mr.  Dakyns  from  the  Scottish 
Highlands. 

3.  State  of  Aggregation. — The  hardness  or  softness  of  a 
rock,  in  other  words,  its  induration,  friability,  or  the  degree 
of  aggregation  of  its  particles,  may  be  either  original  or  ac- 
quired. Some  rocks  (sinters,  for  example)  are  soft  at  first 
and  harden  by  degrees;  the  general  effect  of  exposure,  how- 


11  Trans.  Roy.  Soc.  Edin.  vol.  xxix.  (1879),  p.  504. 

78  Bull.  D.  S.  Geol.  Surv.  No.  66  (1890),  Bull.  Phil.  Soc.  Washington,  xi. 
(1890),  pp.  65,  191,  and  postea,  pp.  457,  458.  Teall  and  Dakyns,  Quart.  Journ. 
Geol.  Soc.  1892. 


188  TEXT-BOOK    OF   GEOLOGY 

ever,  is  to  loosen  the  cohesion  of  the  particles  of  rocks.  A 
rock  which  can  easily  be  scratched  with  the  nail  is  almost 
always  much  decomposed,  though  some  chloritic  and  talcose 
schists  are  soft  enough  to  be  thus  affected.  Compact  rocks 
which  can  easily  be  scratched  with  the  knife,  and  are  ap- 
parently not  decomposed,  may  be  fine-grained  limestones, 
dolomites,  ironstones,  mudstones,  or  some  other  simple 
rocks.  Crystalline  rocks,  except  limestone,  cannot,  as  a 
rule,  be  scratched  with  the  knife  unless  considerable  force 
be  used.  They  are  chiefly  composed  of  hard  silicates,  so 
that  when  an  instance  occurs  where  a  fresh  specimen  can  be 
easily  scratched,  it  will  usually  be  found  to  be  a  limestone 
(pp.  148,  139,  149).  The  ease  with  which  a  rock  may  be 
broken  is  the  measure  of  its  frangibility.  Most  rocks  break 
most  easily  in  one  direction;  attention  to  this  point  will 
sometimes  throw  light  upon  their  internal  structure. 

Fracture  is  the  surface  produced  when  a  rock  is  split 
or  broken,  and  depends  for  its  character  upon  the  texture  of 
the  mass.  Finely  granular,  compact  rocks  are  apt  to  break 
with  a  splintery  fracture  where  wedge-shaped  plates  ad- 
here by  their  thicker  ends  to,  and  lie  parallel  with  the  gen- 
eral surface.  When  the  rock  breaks  off  into  concave  and 
convex  rounded  shell-like  surfaces,  the  fracture  is  said  to 
be  conchoidal,  as  may  be  seen  in  obsidian  and  other 
vitreous  rocks  and  in  exceedingly  compact  limestones.  The 
fracture  may  also  be  f  o  1  i  a  t  e  d,  s  1  a  t  y,  or  s  h  a  1  y,  accord- 
ing to  the  structure  of  the  rock.  Many  opaque,  compact 
rocks  are  translucent  on  the  thin  edges  of  fracture,  and 
afford  there,  with  the  aid  of  a  lens,  a  glimpse  of  their  inter- 
nal composition.  A  rock  is  said  to  be  flinty,  when  it  is 
hard,  close-grained,  and  breaks  with  a  smooth  or  conchoidal 
fracture  like  flint;  friable,  when  it  crumbles  down  like 


GEOGNOSY  189 

dry  clay  or  chalk;  plastic,  when,  like  moist  clay,  it  can 
be  worked  into  shapes;  pulverulent,  when  it  falls  read- 
ily to  powder;  earthy,  when  it  is  decomposed  into  loam 
or  earth;  incoherent  or  loose,  when  its  particles  are 
quite  separate,  as  in  dry  blown  sand. 

4.  Color  and  Lustre* — These  characters  vary  so  much, 
even  in  the  same  rock,  according  to  the  freshness  of  the  sur- 
face examined,  that  they  possess  but  a  subordinate  value. 
Nevertheless,  when  cautiously  used,  color  may  be  made 
to  afford  valuable  indications  as  to  the  probable  nature  and 
composition  of  rocks.  It  is,  in  this  respect,  always  desir- 
able to  compare  a  freshly -broken  with  a  weathered  piece 
of  the  rock.7* 

White  indicates  usually  the  absence  or  a  comparatively 
small  amount  of  the  heavy  metallic  oxides,  especially  iron. 
It  may  either  be  the  original  color,  as  in  chalk  and  calc-sin- 
ter,  or  may  be  developed  by  weathering,  as  in  the  white  crust 
on  flints  and  on  many  porphyries.  Gray  is  a  frequent  color 
of  rocks  which,  if  quite  pure,  would  be  white,  but  which 
acquire  a  grayish  tint  by  admixture  of  dark  silicates,  organic 
matter,  diffused  pyrites,  etc.  Blue  or  bluish-gray  is  a  char- 
acteristic tint  of  rocks  through  which  iron-disulphide  is  dif- 
fused in  extremely  minute  subdivision.  But  as  a  rule  it 
rapidly  disappears  from  such  rocks  on  exposure,  especially 
where  they  contain  organic  matter  also.  The  .stiff  blue  clay 
of  the  sea-bottom  which  is  colored  by  iron-disulphide  be- 
comes reddish-brown  when  dried,  and  then  shows  no  trace 
of  sulphide.74  Black  may  be  due  either  to  the  presence  of 
carbon  (when  weathering  will  not  change  it  much),  or  to 


13  Alterations  of  the  colors  of  minerals  and  rocks  are  effected  by  heat  and 
even  by  sunlight.     See  Janettaz,  Bull.  Soc.  Geol.  xxix.  (1872),  p.  300. 

14  J.  Y.  Buchanan,  Brit.  Assoc.  1881,  p.  584. 


190  TEXT- BOOK    OF   GEOLOGY 

some  iron-oxide  (magnetite  chiefly),  or  some  silicate  rich  in 
iron  (as  hornblende  and  augite).  Many  rocks  (basalts  and 
melaphyres  particularly)  which  look  quite  black  on  a  fresh 
surface,  become  red,  brown  or  yellow  on  exposure,  black 
being  comparatively  seldom  a  weathered  color.  Yellow  (or 
Orange),  as  a  dull  earthy  coloring  matter,  almost  always  in- 
dicates the  presence  of  hydrated  peroxide  of  iron.  In  mod- 
ern volcanic  districts  it  may  be  due  to  iron-chloride,  sulphur, 
etc.  Bright,  metallic,  gold-like  yellow  is  usually  that  of 
iron-disulphide.  Brown  is  the  normal  color  of  some  carbo- 
naceous rocks  (lignite),  and  ferruginous  deposits  (bog-iron- 
ore,  clay-ironstone,  etc.).  It  very  generally,  on  weathered 
surfaces,  points  to  the  oxidation  and  hydration  of  minerals 
containing  iron.  Red,  in  the  vast  majority  of  cases,  is  due 
to  the  presence  of  anhydrous  peroxide  of  iron.  This  min- 
eral gives  dark  blood-red  to  pale  flesh-red  tints.  As  it  is 
liable,  however,  to  hydration,  these  hues  are  often  mixed 
with  the  brown,  orange  and  yellow  colors  of  limonite.76 
Green,  as  the  prevailing  tint  of  rocks,  occurs  among  schists, 
when  its  presence  is  usually  due  to  some  of  the  hydrous 
magnesian  silicates  (chlorite,  talc,  serpentine).  It  appears 
also  among  massive  rocks,  especially  those  of  older  geologi- 
cal formations,  where  hornblende,  olivine,  or  other  silicates 
have  been  altered.  Among  the  sedimentary  rocks,  it  is 
principally  due  to  ferrous  silicate  (as  in  glauconite).  Car- 
bonate of  copper  colors  some  rocks  emerald-  or  verdigris- 
green.  The  mottled  character  so  common  among  many 
stratified  rocks  is  frequently  traceable  to  unequal  weather- 
ing, some  portions  of  the  iron  being  more  oxidized  than 
others;  while  some,  on  the  other  hand,  become  deoxidized 


u  See  I.  C.  Russell,  Bull.  U.  S.  Geol.  Surv.  No.  52  (1889). 


-  GEOGNOSY  191 

from  the  reducing  action  of  decaying  organic  matter,  as  in 
the  circular  green  spots  so  often  found  among  red  strata. 

Lustre,  as  an  external  character  of  rocks,  does  not  pos- 
sess the  value  which  it  has  among  minerals.  In  most  rocks, 
the  granular  texture  prevents  the  appearance  of  any  distinct 
lustre.  A  completely  vitreous  lustre  without  a  granular  tex- 
ture, is  characteristic  of  volcanic  glass.  A  splendent  semi- 
metallic  lustre  may  often  be  observed  upon  the  foliation 
planes  of  schistose  rocks  and  upon  the  laminae  of  micaceous 
sandstones.  As  this  silvery  lustre  is  almost  invariably  due 
to  the  presence  of  mica,  it  is  commonly  called  distinctively 
micaceous.  A  metallic  lustre  is  met  with  sometimes  in  beds 
of  anthracite;  more  usually  its  occurrence  among  rocks  in- 
dicates the  presence  of  metallic  oxides  or  sulphides.  A 
resinous  lustre  is  characteristic  of  many  pitchstones.  Lustre- 
mottling  is  a  term  applied  to  the  interrupted  sheen  on  the 
cleavage  faces  of  minerals  which  have  inclosed  much  smaller 
crystals  or  grains  of  other  minerals.  It  is  well  seen  on  the 
surfaces  of  some  of  the  constituents  of  serpentine  rocks. 

5.  Feel  and  SmelL — These  minor  characters  are  occasion- 
ally useful.  By  the  feel  of  a  mineral  or  rock  is  meant  the 
sensation  experienced  when  the  fingers  are  passed  across  its 
surface.  Thus  hydrous  magnesian  silicates  have  often  a 
marked  soapy  or  greasy  feel.  Some  sericitic  mica-schists 
show  the  same  character.  Trachyte  received  its  name  from 
its  characteristic  rough  or  harsh  feel.  Some  rocks  adhere 
to  the  tongue,  a  quality  indicative  of  their  tendency  to  ab- 
sorb water. 

Smell. — Many  rocks,  when  freshly  broken,  emit  dis- 
tinctive odors.  Those  containing  volatile  hydrocarbons  give 
sometimes  an  appreciable  bituminous  odor,  as  is  the  case 
with  certain  eruptive  rocks,  which  in  central  Scotland  have 


192  TEXT-BOOK   OF   GEOLOGY 

been  intruded  through  coal-seams  and  carbonaceous  shales. 
Limestones  have  often  a  fetid  odor;  rocks  full  of  decompos- 
ing sulphides  are  apt  to  give  a  sulphurous  odor;  those  which 
are  highly  siliceous  yield,  on  being  struck,  an  empyreumatic 
odor.  It  is  characteristic  of  argillaceous  rocks  to  emit  a 
strong  earthy  smell  when  breathed  upon. 

6.  Specific    Gravity* — This    is    an     important    character 
among  rocks  as  well  as  among  minerals.     It   varies  from 
0'6  among  the  hydrocarbon  compounds  to  3-l  among  the 
basalts.     As  already  stated,  the  average  specific  gravity  of 
the  rocks  of  the  earth's  crust  may  be  taken  to  be  about  2-5, 
or  from  that  to  3-0.     Instruments  for  taking  the  specific 
gravity  of  rocks  have  been  already  (p.  154)  referred  to. 

7.  Magnetism  is  so  strongly  exhibited  by  some  crystal- 
line rocks  as  powerfully  to  affect  the  magnetic  needle,  and 
to  vitiate  observations  with  this  instrument.     It  is  due  to 
the  presence  of  magnetic  iron,  the  existence  of  which  may 
be  shown  by  pulverizing  the  rock  in  an  agate  mortar,  wash- 
ing carefully  the  triturated  powder,  and  drying  the  heavy 
residue,  from  which  grains  of  magnetite  or  of  titaniferous 
magnetic  iron  may  be  extracted  with  a  magnet.     This  may 
be  done  with  any  basalt  (p.  156).     A  freely  swinging  mag- 
netic needle  is  of  service,  as  by  its  attraction  or  repulsion  it 
affords  a  delicate  test  for  the  presence  of  even  a  small  quan- 
tity of  magnetic  iron. 

§  v.  Microscopic  Characters  of  Rocks 

No  department  of  Geology  has  been  more  advanced  in 
recent  years  than  Lithology,  and  this  has  been  mainly  due 
to  the  introduction  of  the  microscope  as  an  instrument  for 
investigating  minute  internal  structure.  As  far  back  as  the 
year  1827,  a  method  of  making  thin  transparent  sections  of 


GEOGNOSY  193 

fossil  wood,  and  mounting  them  on  glass  with  Canada  bal- 
sam, had  been  devised  by  William  Nicol  of  Edinburgh,  and 
was  employed  by  Henry  Witham  in  his  "History  of  Fossil 
Vegetables."  76 

It  was  not,  however,  until  1856  that  Mr.  H.  C.  Sorby, 
applying  this  method  to  the  investigation  of  minerals  and 
rocks,  showed  how  many  and  important  were  the  geological 
questions  on  which  it  was  calculated  to  shed  light."  Refer- 
ence will  be  made  in  subsequent  pages  to  the  remarkable 
results  then  announced  by  him.  To  the  publication  of  his 
memoir  the  subsequent  rapid  development  of  the  micro- 
scopic study  of  rocks  may  be  distinctly  traced.  The  micro- 
scopic method  of  analysis  is  now  in  use  in  every  country 
where  attention  is  paid  to  the  history  of  rocks.'8 

In  §  iii.  p.  161  information  has  been  given  regarding  the 


16  Small  4to,  Edinburgh,  1831.  This  work,  though  dedicated  to  Nicol,  does 
not  distinctly  recognize  him  as  the  actual  inventor  of  the  process  of  slicing  min- 
eral substances  for  microscopic  investigation.  All  that  was  original  in  Witham'a 
researches  he  owed  either  directly  or  indirectly  to  Nicol. 

11  Brit.  Assoc.  1856,  Sect.  p.  78.  Quart.  Jourii.  Geol.  Soc.  xiv.  1858.  Micr. 
Journ.  xvii.  (1877),  p.  113. 

18  Among  the  best  text-books  on  this  subject  the  following  may  be  mentioned : 
— "Mikroskopische  Beschaffenheit  der  Miner-alien  und  Gesleine,"  F.  Zirke), 

1  vol.    1873.     "Mikroskopische  Physiographic  der  Mineralien  und  Gesteiue," 
H.  Rosenbusch,  2  vols.   2d  Edit.  1885-87,  and  the  English  translation  of  the 
first  volume  quoted  on  p.  161;  likewise  the  Tables  translated  by  F.    H.  Hatch 
quoted  on  p.  161.     "Elemente  der  Petrographie,"  A.  von  Lasaulx,  1875.     "Min- 
eralogie  microgaphique:  roches  eruptives  francaises,"  Fouque  and  Michel-Levy, 

2  vols.  4to,  Paris,  1879.      "Microscopical  Petrography,"  Zirkel,  being  vol.  vi.  of 
ihe  Geol.  Explor.  of  40th  Parallel,  Washington,  1876.     "British  Petrography," 
J.  J.  H.  Teall,  London,  1888.     "Les  Mineraux  des  Roches,"  Michel-Levy  and 
Lacroix,  Paris,  1888.     The  volumes  for  the  last  fifteen  or  twenty  years  of  the 
Quarterly  Journal  of  the  Geological  Society,  Geological  Magazine,  Neues  Jahr- 
buch  fiir  Mineralogie,  etc.,  Zeitschrift  der  Deutschen  Geologischen  Gesellschaft, 
Bulletin  de  la  Societe  geologique  de  France,,  Jahrbuch  der  K.  K.  Geologischen 
Reichsanstalt  (Vienna),  contain  numerous  papers  on  the  microscopic  structure 
of  rocks.    Rutley's  "Study  of  Rocks,"  1879,  and  his  "Rock-forming  Minerals," 
1888;  Cole's  "Aids  in  Practical  Geology,"  1891;   and  Hatch's  "Petrology — 
Igneous  Rocks,"  1891,  are  useful  handbooks.     The  manual  of  Rosenbusch  and 
the  work  of  Fouque  aud  Michel-Levy,  contain  a  tolerably  ample  bibliography  of 
the  subject,  to  which  the  student  is  referred.     The  titles  of  some  of  the  more 
important  memoirs  which  have  recently  appeared  will  be  given  in  footnotes. 

GEOLOGY— Vol.  XXIX— 9 


194  TEXT-BOOK   OF   GEOLOGY 

preparation  of  sections  of  rocks  for  microscopical  examina- 
tion, the  methods  of  procedure  in  the  practice  of  this  part 
of  geological  research,  and  some  of  the  terms  employed  in 
the  following  pages. 

1.  Microscopic  Elements  of  Rocks 

Rocks  when  examined  in  thin  sections  with  the  micro- 
scope are  found  to  be  composed  of  or  to  contain  various  ele- 
ments, of  which  the  more  important  are,  1st,  crystals,  or 
crystalline  substances;  2d,  glass;  3d,  crystallites;  4th,  de- 
tritus. 

A.  CRYSTALS  OR  CRYSTALLINE  SUBSTANCES.— Rock- 
forming  minerals,  when  not  amorphous,  may  be  either 
crystallized  in  their  proper  crystallographic  forms  (idio- 
morphic),  or  while  possessing  a  crystalline  internal  struc- 
ture, may  present  no  definite  external  geometrical  form 
(allotriomorphic,  p.  209).  The  latter  condition  is  more 
prevalent,  seeing  that  minerals  have  usually  been  devel- 
oped round  and  against  each  other,  thus  mutually  hinder- 
ing the  assumption  of  determinate  crystallographic  contours. 
Other  causes  of  imperfection  are  fracture  by  movement  in 
the  original  magma  of  the  rock,  and  partial  solution  in  that 
magma  (Fig.  12),  as  in  the  corroded  quartz  of  quartz-por- 
phyries and  rhyolites,  and  the  hornblende  crystals  of  basalts. 
The  ferro-magnesian  minerals  of  earlier  consolidation  among 
basalts  and  andesites,  are  sometimes  surrounded  with  a  dark 
shell  called  the  corrosion-zone.  In  some  rocks,  such  as 
granite,  the  thoroughly  crystalline  character  of  the  com- 
ponent ingredients  is  well  marked,  yet  they  less  frequently 
present  the  definite  isolated  crystals  so  often  to  be  observed 
in  porphyries  and  in  many  old  and  modern  volcanic  rocks. 
Among  thoroughly  crystalline  rocks,  good  crystals  of  the 


GEOGNOSY  195 

component  minerals  may  be  obtained  from  fissures  and  cavi- 
ties in  which  there  has  been  room  for  their  formation.  It  is 
in  the  "drusy"  cavities  of  granite,  for  example,  that  the 
well-defined  prisms  of  felspar,  quartz,  mica,  topaz,  beryl 
and  other  minerals  are  found.  Successive  stages  in  order 
of  appearance  or  development  can  readily  be  observed 
among  the  crystals  of  rocks.  Some  appear  as  large,  but 
frequently  broken,  or  corroded  forms.  These  have  evi- 
dently been  formed  first.  Others  are  smaller  but  abundant, 
usually  unbroken,  and  often  disposed  in  lines.  Others  have 
been  developed  by  subsequent  alteration  within  the  rock.7' 

A  study  of  the  internal  structure  of  crystals  throws  light 
not  merely  on  their  own  genesis,  but  on  that  of  the  rocks  of 
which  they  form  part,  and  is  therefore  well  worthy  of  the 
attention  of  the  geologist.  That  many  apparently  simple 
crystals  are  in  reality  compound,  may  not  infrequently  be 
detected  by  the  different  condition  of  weathering  in  the  two 
opposite  parts  of  a  twin  on  an  exposed  face  of  rock.  The 
internal  structure  of  a  crystal  modifies  the  action  of  solvents 
on  its  exterior  (e.g.  weathered  surfaces  of  calcite,  aragonite 
and  felspars).  Crystals  may  occasionally  be  observed  built 
up  of  rudimentary  "microlites,"  as  if  these  were  the  simplest 
forms  in  which  the  molecules  of  a  mineral  begin  to  appear 
(p.  205). 

A  microscopic  examination  of  some  rocks  shows  that  a 
subsequent  or  secondary  growth  of  different  minerals  has 
taken  place  after  their  original  crystalline  form  was  com- 
plete. These  later  additions  are  in  optical  continuity  with 
the  original  crystal,  and  sometimes  have  taken  place  even 
upon  worn  or  imperfect  forms.  They  may  be  occasionally 
detected  among  the  silicates  of  igneous  rocks,  and  also  even 

19  Fouque  and  Michel-Levy,  "Min.  Micrograph."  p.  161. 


196  TEXT-BOOK   OF   GEOLOGY 

among  the  sandgrains  of  sandstones  which  have  thus  had 
their  rounded  forms  converted  into  crystallographic  faces.80 

Crystalline  minerals  are  seldom  free  from  extraneous  in- 
clusions. These  are  occasionally  large  enough  to  be  readily 
seen  by  the  naked  eye.  But  the  microscope  reveals  them 
in  many  minerals  in  almost  incredible  quantity.  They 
are,  a,  vesicles  containing  gas;  /?,  vesicles  containing  liquid; 
f,  globules  of  glass  or  of  some  lithoid  substance;  <J,  crys- 
tals; e,  filaments,  or  other  indefinitely-shaped  pieces,  patches, 
or  streaks  of  mineral  matter. 

a.  Gr as-filled  cavities — are  most  frequently  globu- 
lar or  elliptical,  and  appear  to  be  due  to  the  presence  of  gas 
or  steam  in  the  crystal  at  the  time  of  consolidation.  Zirkel 
estimates  them  at  360,000,000  in  a  cubic  millimetre  of  the 
hauyne  from  Melfi.81  In  some  instances  the  cavity  has  a 
geometric  form  belonging  to  the  crystalline  system  of  the 
inclosing  mineral.  Such  a  space  defined  by  crystallographic 
contours  is  a  negative  crystal.  A  cavity  filled  with  gas  con- 
tains no  bubble,  and  its  margin  is  marked  by  a  broad  dark 
band.  The  usual  gas  is  nitrogen,  with  traces  of  oxygen 
and  carbon-dioxide;  sometimes  it  is  entirely  carbon-dioxide 
or  hydrogen  and  hydrocarbons. 

y3.  Vesicles  containing  liquid  (and  gas). — As 
far  back  as  the  year  1823,  Brewster  studied  the  nature  of 
certain  fluid-bearing  cavities  in  different  minerals.62  The 


80  H.  C.  Sorby,  Presidential  Address,  Geol.  Soc.  1880,  p.  62.  R.  D.  Irving 
and  C.  R.  Van  Hise  "On  secondary  enlargements  of  Mineral  Fragments  in  cer- 
tain rocks."  Bull.  U.  S.  Geol.  Surv.  No.  8  (1884).  J.  W.  Judd,  Quart.  Journ. 
Geol.  Soc.  xlv.  (1889),  p.  175. 

si  "Mik.  Beschaff."  p.  86. 

*2  Edin.  Phil.  Journ.  ix.  p.  94.  Trans.  Roy.  Soc.  Edin.  x.  p.  1.  See  also 
W.  Nicol,  Edin.  New  Phil.  Journ.  (1828),  v.  p.  94;  De  la  Vallee  Poussin  and 
Renard,  Acad.  Roy.  Belg.  1876,  p.  41 ;  Hartley,  Journ.  Chem.  Soc.  ser.  2,  xiv. 
137;  ser.  3,  ii.  p.  241;  Microscop.  Journ.  xv.  p.  170;  Brit.  Assoc.  1877,  Sect. 
p.  232. 


GEOGNOSY  197 

first  observer  who  showed  their  important  bearing  on  geo- 
logical researches  into  the  origin  of  crystalline  rocks  was 
Mr.  Sorby,  in  whose  paper,  already  cited,  they  occupy  a 
prominent  place.  Vesicles  entirely  filled  with  liquid  are 
distinguished  by  their  sharply-defined  and  narrow  black 
borders.  Vesicular  spaces  containing  fluid  may  be  noticed 
in  many  artificial  crystals  formed  from  aqueous  solutions 
(crystals  of  common  salt  show  them  well)  and  in  many  min- 
erals of  crystalline  rocks.  They  are  exceedingly  various  in 
form,  being  branching,  curved,  oval,  or  spherical,  and  some- 
times assuming  as  negative  crystals  a  geometric  form,  like 
that  characteristic  of  the  mineral  in  which  they  occur,  as 
cubic  in  rock-salt  and  hexagonal  in  quartz.  They  also  vary 
greatly  in  size.  Occasionally  in  quartz,  sapphire,  and  other 
minerals,  large  cavities  are  readily  observable  with  the  naked 
eye.  But  they  may  be  traced  with  high  magnifying  powers 
down  to  less  than  ]0J00  of  an  inch  in  diameter.  Their  propor- 
tion in  any  one  crystal  ranges  within  such  wide  limits,  that 
whereas  in  some  crystals  of  quartz  few  may  be  observed,  in 
others  they  are  so  minute  and  abundant  that  many  millions 
must  be  contained  in  a  cubic  inch.  The  fluid  present  is 
usually  water,  frequently  with  saline  solutions,  particularly 
chloride  of  sodium  or  of  potash,  or  sulphates  of  potash, 
soda,  or  lime.  Carbon-dioxide  may  be  present  in  the  water; 
sometimes  the  cavities  are  partially  occupied  with  it  in 
liquid  form,  and  the  two  fluids,  as  originally  observed  by 
Brewster,  may  be  seen  in  the  same  cavity  unmingled,  the 
carbon-dioxide  remaining  as  a  freely  moving  globule  within 
the  carbonated  water.  Cubic  crystals  of  chloride  of  sodium 
may  be  occasionally  observed  in  the  fluid,  which  must  in 
such  cases  be  a  saturated  solution  of  this  salt  (Fig.  11,  low- 
est figure  in  Column  A).  Usually  each  cavity  contains  a 


198  TEXT-BOOK    OF   GEOLOGY 

small  globule  or  bubble,  sometimes  stationary,  sometimes 
movable  from  one  side  or  end  of  the  cavity  to  the  other,  as 
the  specimen  is  turned.  With  a  high  magnifying  power, 
the  minuter  bubbles  may  be  observed  to  be  in  motion,  some- 
^^  times  slowly  pulsating  from 
&JS  (2^2  side  to  side,  or  rapidly  vi- 
*?&  ^/  l8^!^  brating  like  a  living  organ- 

^^  ^^          ^^     ism.       The    cause    of    this 

*^         H*^       £r  trepidation,    which    resem- 

£j          4P      bles  the  so-called  "Brown- 
ian  movements,"  has  been 
Fig.  ii.-cavities  in  Crystals,  highly  mag-  plausibly  explained  by  the 

nified;    A,    Liquid    Inclusions;    B,    Glass    .  ,  ... 

Inclusions;    c,  Cavities  showing  the  de-    inCCSSant  interchange  OI   the 
vitrification  of  the  original  glass  by  the 

•••  in  the  molecules   from  the 


to  the  vaporous  condition 
along  the  surface  where  vapors  and  liquid  meet — an  inter- 
change which,  though  not  visible  on  the  large  bubbles,  makes 
itself  apparent  in  the  minute  examples,  of  which  the  dimen- 
sions are  comparable  to  those  of  the  intermolecular  spaces.83 
The  bubble  may  be  made  to  disappear  by  the  application  of 
heat. 

With  regard  to  the  origin  of  the  bubble,  Sorby  pointed 
out  that  it  can  be  imitated  in  artificial  crystals,  in  which 
he  explained  its  existence  by  diminution  of  volume  of  the 
liquid  owing  to  a  lowering  of  temperature  after  its  inclosure. 
By  a  series  of  experiments  he  ascertained  the  rate  of  expan- 
sion of  water  and  saline  solutions  up  to  a  temperature  of 
200°  C.  (392°  Fahr.),  and  calculated  from  them  the  tempera- 
ture at  which  the  liquid  in  crystals  would  entirely  fill  its 


83  Charbonelle  and  Thirion,  Rev.  Quest.  Scientif.  vii.  (1880)43.  On  the  criti- 
cal point  of  water,  etc.,  in  these  cavities  see  Hartley,  Journ.  Chem.  Soc.  ser.  3, 
vol.  ii.  p.  241.  Pop.  Sci.  Rev.  new  ser.  i.  p.  119. 


GEOGNOSY  199 

inclosing  cavities.  Thus,  in  the  nepheline  of  the  ejected 
blocks  of  Monte  Somma,  he  found  that  the  relative  size  of 
the  vacuities  was  about  -28  of  the  fluid,  and  assuming  the 
pressure  under  which  the  crystals  were  formed  to  have  been 
not  much  greater  than  sufficient  to  counteract  the  elastic 
force  of  the  vapor,  he  concluded  that  the  nepheline  may 
have  been  formed  at  a, temperature  of  about  340°  C.  (644° 
Fahr.),  or  a  very  dull  red  heat,  only  just  visible  in  the 
dark.  He  estimated  also  from  the  fluid  cavities  in  the 
quartz  of  granite  that  this  rock  has  probably  consolidated 
at  somewhat  similar  temperatures,  under  a  pressure  some- 
times equal  to  that  of  76,000  feet  of  rock.84  Zirkel,  however, 
has  pointed  out  that  even  in  contiguous  cavities,  where  there 
is  no  evidence  of  leakage  through  fine  fissures,  the  relative 
size  of  the  vacuole  varies  within  very  wide  limits,  and  in 
such  a  manner  as  to  indicate  no  relation  whatever  to  the 
dimensions  of  the  inclosing  cavities.  Had  the  vacuole  been 
due  merely  to  the  contraction  of  the  liquid  on  cooling,  it 
ought  to  have  always  been  proportionate  to  the  size  of  the 
cavity.86 

MM.  De  la  Valle'e  Poussin  and  Renard,  attacking  the 
question  from  another  side,  measured  the  relative  dimen- 
sions of  the  vesicle  and  of  its  inclosed  water  and  cube  of 
rock-salt,  as  contained  in  the  quartziferous  diorite  of 
Quenast  in  Belgium.  The  temperature  at  which  the  asper- 
tained  volume  of  water  in  the  cavity  would  dissolve  its 
salt  was  found  by  calculation  to  be  307°  C.  (520°  Fahr.). 
But  as  the  law  of  the  solubility  of  common  salt  has  not 
been  experimentally  determined  for  high  temperatures,  this 
figure  can  only  be  accepted  provisionally,  though  other 


84  Sorby,  Q.  J.  Geol.  Soc.  xiv.  pp.  480,  493.        <*  "Mik.  Beschaff."  p.  46. 


200  TEXTBOOK    OF   GEOLOGY 

considerations  go  to  indicate  that  it  is  probably  not  far 
from  the  truth.  Assuming  then  that  this  was  the  tempera- 
ture at  which  the  vesicle  was  formed,  these  authors  proceed 
to  determine  the  pressure  necessary  to  prevent  the  complete 
vaporization  of  the  water  at  that  temperature,  and  obtain,  as 
the  result,  a  pressure  of  87  atmospheres,  equal  to  84  tons 
per  square  foot  of  surface.8*  That  many  rocks  were  formed 
under  great  pressure  is  well  shown  by  the  liquid  carbon- 
dioxide  in  the  pores  of  their  crystals. 

Although,  in  almost  all  cases,  the  liquid  inclusions  are 
to  be  referred  to  the  conditions  under  which  the  mineral 
crystallized  out  of  the  original  magma,  they  may  be  excep- 
tionally developed  long  subsequently,  either  in  one  of  the 
original  minerals  during  decomposition,  or  in  a  mineral  of 
secondary  origin,  such  as  quartz  of  subsequent  introduc- 
tion.87 

Liquid  inclusions  may  be  dispersed  at  random  through 
a  crystal,  or  as  in  the  quartz  of  granite,  gathered  in  inter- 
secting planes  (which  look  like  fine  fissures  and  which  may 
sometimes  have  become  real  fissures,  owing  to  the  line  of 
weakness  caused  by  fche  crowding  of  the  cavities),  or  dis- 
posed regularly  in  reference  to  the  contour  of  the  crystal. 
In  the  last  case  they  are  sometimes  confined  to  the  centre, 
sometimes  arranged  in  zones  along  the  lines  of  growth  of 
the  crystal.68  They  are  specially  conspicuous  in  the  quartz 


88  "Memoire  snr  lea  Roches  dites  Plutoniennea  de  la  Belgique,"  De  la  Vallee 
Poussin  and  A.  Renard.  Acad.  Roy.  Belg.  1876,  p.  41.  See  also  Ward,  Q.  J. 
Geol.  Soc.  xxxi.  p.  568,  who  believed  that  the  granites  of  Cumberland  consoli- 
dated at  a  maximum  depth  of  22,000  to  30,000  feet. 

81  See  Whitman  Cross  on  the  development  of  liquid  inclusions  in  plagioclase 
during  the  decomposition  of  the  gneiss  of  Brittany.  Tschermak's  Min.  MittheiL 
1880,  p.  369;  also  G.  F.  Becker,  "Geology  of  Comstock  Lode."  U.  S.  Geol. 
Surv.  1882,  p.  371. 

88  The  way  in  which  vesicles,  inclosed  crystals,  etc.,  are  grouped  along  the 
zones  of  growth  of  crystals  is  illustrated  in  Fig.  12. 


GEOGNOSY  201 

of  granite  and  other  massive  rocks,  as  well  as  of  gneiss  and 
mica-schist;  also  in  felspars,  topaz,  beryl,  augite,  nepheline, 
olivine,  leucite  and  other  minerals. 

f.  Inclusions  of  glass  or  of  some  lithoid 
substance. — In  many  rocks  which  have  consolidated 
from  fusion,  the  component  crystals  contain  globules  or 
irregularly  shaped  inclosures  of  a  vitreous  nature  (Fig.  11, 
Column  B).  These  inclosures  are  analogous  to  the  fluid- 
inclusions  just  described.  They  are  portions  of  the  original 
glassy  magma  out  of  which  the  minerals  of  the  rock  crystal- 
lized, as  portions  of  the  mother-liquor  are  inclosed  in  artifi- 
cially formed  crystals  of  common  salt.  That  magma  is  in 
reality  a  liquid  at  high  temperatures,  though  at  ordinary 
temperatures  it  becomes  a  solid.  At  first,  these  glass- 
vesicles  may  be  confounded  with  the  true  liquid-cavities, 
which  in  some  respects  they  closely  resemble.  But  they 
may  be  distinguished  by  the  immobility  of  their  bubbles, 
of  which  several  are  sometimes  present  in  the  same  cavity; 
by  the  absence  of  any  diminution  of  the  bubbles  when  heat 
is  applied;  by  the  elongated  shape  of  many  of  the  bubbles; 
by  the  occasional  extrusion  of  a  bubble  almost  beyond  the 
walls  of  the  vesicle;  by  the  usual  pale  greenish  or  brownish 
tint  of  the  substance  filling  the  vesicle,  and  its  identity 
with  that  forming  the  surrounding  base  or  ground-mass 
in  which  the  crystals  are  imbedded;  and  by  the  com- 
plete passivity  of  the  substance  in  polarized  light  (see 
p.  169). 

Glass  inclusions  occur  abundantly  in  some  minerals, 
aggregated  in  the  centre  of  a  crystal  or  ranged  along  its 
zones  of  growth  with  singular  regularity.  They  appear  in 
felspars,  quartz,  leucite,  and  other  crystalline  ingredients 
of  volcanic  rocks,  and  of  course  prove  that  in  such  posi- 


202  TEXT-BOOK   OF   QEOLOGY 

tions  these  minerals,  even  the  refractory  quartz,  have 
undoubtedly  crystallized  out  of  molten  solutions. 

In  inclusions  of  a  truly  vitreous  nature,  traces  of  de- 
vitrification may  not  infrequently  be  seen.  In  particular, 
microscopic  crystallites  (p.  205)  make  their  appearance,  like 
those  in  the  ground-mass  of  the  rock.  Sometimes  the  in- 
clusions, like  the  general  ground-mass,  have  an  entirely 
stony  character  (Fig.  11,  C).  This  may  be  well  observed 
in  those  which  have  not  been  entirely  separated  from  the 
surrounding  ground-mass,  but  are  connected  with  it  by  a 
narrow  neck  at  the  periphery  of  the  inclosing  crystal,  la 
some  granites  and  in  elvans,  the  quartz  by  irregular  contrac- 
tion, while  still  .in  a  plastic  state,  appears  to  have  drawn 
into  its  substance  portions  of  the  surrounding  already 
lithoid  base;89  but  this  appearance  may  sometimes  be  due 
to  irregular  corrosion  of  the  crystals  by  the  magma.90 

d.  Crystals  and  crystalline  bodies. — Many  com- 


Fig.  12.— Section  of  a  fractured  and  corroded  Augite  crystal 
from  a  dike,  Crawfordjohn,  Lanarkshire  (magnified),  showing 
lines  of  growth  with  vesicles  and  magnetite  crystals. 

ponent  minerals  of  rocks  contain  other  minerals  (Fig.  12). 
These   occur  sometimes   as   perfect  crystals,   more  usually 

89  J.  A.  Phillips,  Q.  J.  Geol.  Soc.  xxxi.  p.  338. 

90  Fouque  and  Michel-Levy,  "Min.  Micrograph." 


GEOGNOSY  203 

as  what  ar.e  termed  microlites  (p.  205).  Like  the  glass- 
inclusions,  they  tend  to  range  themselves  in  lines  along 
the  successive  zones  of  growth,  in  the  inclosing  mineral. 
Microlites  are  of  frequent  occurrence  in  leucite,  garnet, 
augite,  hornblende,  calcite,  fluorite,  etc.  From  the  fact 
that  microlites  of  the  easily  fusible  augite  are,  in  the 
Vesuvian  lavas,  inclosed  within  the  extremely  refractory 
leucite,  it  was  supposed  that  the  relative  order  of  fusibility 
is  not  always  followed  in  the  microlites  and  enveloping 
crystals.  But  this  has  been  satisfactorily  explained  by 
Fouque"  and  Michel-Levy,  who  have  shown  experimentally 
that  leucite,  when  crystallizing  from  fusion,  tends  to  catch 
up  inclusions  of  the  surrounding  glass,  which,  should  the 
glass  be  pyroxenic,  may  assume  the  form  of  augite.91 

e.  Filaments,  streaks,  patches,  discolorations. 
— Besides  the  inclosures  already  enumerated,  crystals  like- 
wise frequently  inclose  irregular  portions  of  mineral  matter, 
due  to  alteration  of  the  original  substance  of  the  minerals 
or  rocks.  Thus  tufts  and  vermicular  aggregates  of  certain 
green  ferruginous  silicates  are  of  common  occurrence  among 
the  crystals  and  cavities  of  old  pyroxenic  volcanic  rocks. 
Orthoclase  crystals  are  often  mottled  with  patches  of  a  gran- 
ular nature,  due  to  partial  conversion  of  the  mineral  into 
kaolin.  The  magnetite,  so  frequently  inclosed  within  min- 
erals, is  abundantly  oxidized,  and  has  given  rise  to  brown 
and  yellow  patches  and  discolorations.  Care  must  be  taken 
not  to  confound  these  results  of  infiltrating  water  with  the 
original  characters  of  a  rock.  Practice  will  give  the  student 
confidence  in  distinguishing  them,  if  he  familiarizes  his  eye 
with  decomposition  products  by  studying  slices  of  weathered 
minerals  and  of  the  weathered  parts  of  rocks. 

91  "Synthese  des  Mineraux,"  1882,  p.  156. 


204  TEXT-BOOK    OF   GEOLOGY 

B.  GLASS.— Even  to  the  unassisted  eye,  many  volcanic 
rocks  consist  obviously  in  whole  or  in  great  measure  of 
glass.92  This  substance  in  mass  is  usually  black  or  dark 
green,  but  when  examined  in  thin  sections  under  the  micro- 
scope, it  presents  for  the  most  part  a  pale  brown  tint,  or  is 
nearly  colorless.  In  its  purest  condition,  it  is  quite  struc- 
tureless, that  is,  it  contains  no  crystals,  crystallites,  or  other 
distinguishable  individualized  bodies.  But  even  in  this 
state  it  may  sometimes  be  observed  to  be  marked  by  clot- 
like  patches  or  streaks  of  darker  and  lighter  tint,  arranged 
in  lines  or  eddy-like  curves,  indicative  of  the  flow  of  the 
original  fluid  mass.  Rotated  in  the  dark  field  of  crossed 
Nicol-prisms,  such  a  natural  glass  remains  dark,  as,  unless 
where  it  has  undergone  internal  stresses,  it  is  perfectly  inert 
in  polarized  light.  Being  thus  isotropic,  it  may  readily  be 
distinguished  from  any  inclosed  crystals  which,  acting  on 
the  light,  are  anisotropic  (p.  169).  Perfectly  homogeneous 
structureless  glass,  without  inclosures  of  any  kind,  occurs 
for  the  most  part  only  in  limited  patches,  even  in  the  most 
thoroughly  vitreous  rocks.  Originally  the  structure  of  all 
glassy  rocks,  at  the  time  of  most  complete  fusion,  may  have 
been  that  of  perfectly  unindividualized  glass.  But  as  these 
masses  tended  toward  a  solid  form,  devitrification  of  their 
glass  set  in.  Many  forms  of  incipient  or  imperfect  crystal- 
lization, as  well  as  perfect  crystals,  were  developed  in  the 
still  fluid  and  moving  mass,  and,  together  with  crystals  of 
earlier  growth,  were  arranged  in  the  direction  of  motion. 
Devitrification  has  in  frequent  examples  proceeded  so  far 
that  no  trace  remains  of  any  actual  glass.93 

w  See  B.  Cohen  on  glassy  Rocks.     Neues  .Tahrb.  1880  (ii.),  p.  23. 

93  Consult  a  paper  on  the  microscopic  character  of  devitrified  glass  and  some 
analogous  rock-structures,  by  D.  Herman  and  F.  Rutley.  Proc.  Roy.  Soc.  1885, 
p.  87. 


GEOGNOSY  205 

C.  CRYSTALLITES  AND  MICROLITES.'* —  Under  these 
names  may  be  included  minute  inorganic  bodies  possess- 
ing a  more  or  less  definite  form,  but  generally  without  the 
geometrical  characters  of  crystals.  They  occur  most  com- 
monly in  rocks  which  have  been  formed  from  igneous  fusion, 
but  are  found  also  in  others  which  have  resulted  from,  or 
have  been  altered  by,  aqueous  solutions.  They  seem  to  be 
early  or  peculiar  forms  of  crystallization.  They  are  abun- 
dantly developed  in  artificial  slags,  and  appear  in  many 
modern  and  ancient  vitreous  rocks,  but  the  conditions  under 
which  they  are  produced  are  not  yet  well  understood.96 

Crystallites  are  disti nguished  by  remaining  isotropic 
in  polarized  light.  The  simplest  are  extremely  minute  drop- 
like  bodies  or  globulites,  sometimes  crowded  confusedly 
through  the  glass,  giving  it  a  dull  or  somewhat  granular 
character,  while  in  other  cases  they  are  arranged  in  lines  or 
groups.  Gradations  can  be  traced  from  spherical  or  sphe- 
roidal globulites  into  other  forms  more  elliptical  in  shape, 
but  still  having  a  rounded  outline  and  sometimes  sharp  ends 
(longulites).  There  does  not  appear  to  be  any  essential  dis- 
tinction, save  in  degree  of  development,  between  these  forms 
and  the  long  rod-like  or  needle-shaped  bodies  which  have 
been  termed  belonites.  Existing  sometimes  as  mere  simple 
needles  or  rods,  these  more  elongated  crystallites  may  be 
traced  into  more  complex  forms,  curved  or  coiled,  at  one 


94  The  word  crystallite  was  first  used  by  Sir  James  Hall  to  denote  the  lithoid 
substance  obtained  by  him  after  fusing  and  then  slowly  cooling  various  "whin- 
stones"  (diabases,  etc.).     Since  its  revival  in  lithology  it  has  been  applied  to 
the  minuter  bodies  above  described.     The  student  should  consult  Vogelsang's 
"Philosophic  der  Geologic,"  p.   139;  "Krystalliten, "  Bonn,   8vo,   1875;    also 
his  descriptions   in  Archives  Neerlandaises,  v.   1870,  vi.   1871.      Sorby,  Brit. 
Assoc.  1880. 

95  They  are  well  exhibited  also  in  ordinary  blow-pipe  beads.     See  Sorby, 
Brit.  Assoc.  1880,  or  Geol.  Mag.  1880,  p.  468.     They  have  been  produced  ex- 
perimentally in  the  artificial  rocks  fused  by  Messrs.  Fouque  and  Michel-Le"vy. 


206 


TEXT-BOOK    OF    GEOLOGY 


time  solitary,  at  another  in  groups.  In  most  cases,  crystal- 
lites are  transparent  and  colorless,  or  slightly  tinted,  but 
sometimes  they  are  black  and  opaque,  from  a  coating  of  fer- 
ruginous oxide,  or  only  appear  so  as  an  optical  delusion 
from  their  position.  Black,  seemingly  opaque,  hair- like, 
twisted  and  curved  forms,  termed  trichites,  occur  abundantly 
in  obsidian. 

Mi.crolites  are  other  incipient  forms  of  crystallization 
which  differ  from  crystallites  in  that  they  react  on  polarized 
light.  They  assume  rod -like  or  needle-shaped  forms,  some- 
times occurring  singly,  sometimes  in  aggregates,  and  even 
occasionally  grouped  into  skeleton-crystals.  They  can  fofr 
the  most  part  be  identified  as  rudimentary  forms  of  definite 
minerals  such  as  augite,  hornblende,  felspar,  olivine,  and 
magnetite. 

Good  illustrations  of  the  general  character  and  grouping 


Fig-.  13. — Augite  Crystal  surrounded 
by  Crystallites  and  Microlites. 
from  the  vitreous  Andesite  of 
Eskdalemuir,  magnified  800  Di- 
ameters. 


Fig.  14.— Microlites  of  the  Pitch- 
stone  of  Arran,  magnified  70 
Diameters.  (See  p.  283  ) 


of  crystallites  and  rnicrolites  are  shown  in  some  vitreous 
basalts.  In  Fig.  13,  for  example,  the  outer  portion  of  the 
field  displays  crowded  globulites  and  longulites  as  well  as 
here  and  there  a  few  belonites  and  some  curved  and  coiled 
trichites.  Bound  the  rude  augite  crystal,  these  various 


GEOGNOSY  207 

bodies  have  been  drawn  together  out  of  the  surrounding 
glass.  Numerous  rod-like  microlites  diverge  from  the  crys- 
tal, and  these  are  more  or  less  thickly  crusted  with  the  sim- 
pler and  smaller  forms."  In  Fig.  14,  the  remarkably  beau- 
tiful structure  of  an  Arran  pitchstone  is  shown;  the  glassy 
base  being  crowded  with  minute  microlites  of  hornblende 
which  are  grouped  in  a  fine  feathery  or  brush-like  arrange- 
ment round  tapering  rods.  In  this  case,  also,  we  see  that 
the  glassy  base  has  been  clarified  round  the  larger  individ- 
uals by  the  abstraction  of  the  crowded  smaller  microlites. 
By  the  progressive  development  of  crystallites,  microlites, 
or  crystals  during  the  cooling  and  consolidation  of  a  molten 
rock,  a  glass  loses  its  vitreous  character  and  becomes  lithoid; 
in  other  words,  undergoes  devitrification. 

The  characteristic  amorphous  or  indefinitely  granular 
and  fibrous  or  scaly  matter,  constituting  the  microscopic 
base  in  which  the  definite  crystals  of  felsites  and  porphyries 
are  imbedded  (pp.  278-281),  has  been  the  subject  of  much 
discussion.  Between  crossed  Nicol-prisms  it  sometimes  be- 
haves isotropically,  like  a  glass,  but  in  other  cases  allows  a 
mottled  glimmering  light  to  pass  through.  It  is  now  well 
understood  to  be  a  product  of  the  devitrification  of  once 
glassy  rocks  wherein  the  crystallitic  and  microlitic  forms 
can  still  be  recognized  or  have  been  more  or  less  effaced  by 
subsequent  alteration  by  infiltrating  water.'7  ,, 

Every  gradation  in  the  relative  abundance  of  crystallites 
may  be  traced.  In  some  obsidians  and  other  vitreous  rocks, 
portions  of  the  glass  can  be  obtained  with  comparatively 
few  of  them;  but  in  the  same  rocks  we  may  not  infrequently 


96  Proc.  Eoy.  Phys.  Soc.  Edin.  v.  p.  246,  Fig.  5.     J.  J.  H.  Teall,  Q.  J.  GeoL 
Soc.  xl.  p.  221,  Plate  xii.  Fig.  2a. 

91  See  Zirkel,  "Mik.  BeschafE."  p.  280.     Rosenbusch,  voL  ii.  p.  60. 


208  TEXT-BOOK    OF   GEOLOGY 

observe  adjacent  parts  where  they  have  been  so  largely  de- 
veloped as  to  usurp  the  place  of  the  original  glass,  and  give 
the  rock  in  consequence  a  lithoid  aspect  (Fig.  11,  C  and 
pp.  278-283). 

D.  DETRITUS. — Many  rocks  are  composed  of  the  detritus 
of  pre-existing  materials.  In  the  great  majority  of  cases 
this  can  be  readily  detected,  even  with  the  naked  eye.  But 
where  the  texture  of  such  detrital  or  fragmental  (clastic) 
rocks  becomes  exceedingly  fine,  their  true  nature  may  re- 
quire elucidation  with  the  microscope  (Figs.  21,  22).  An 
obvious  distinction  can  be  drawn  between  a  mass  of  com- 
pact detritus  and  a  crystalline  or  vitreous  rock.  The  de- 
trital materials  are  found  to  consist  of  various  and  irregu- 
larly shaped  grains,  with  more  or  less  of  an  amorphous  and 
generally  granular  paste.  In  some  cases,  the  grains  are 
broken  and  angular,  in  others  they  are  rounded  or  water- 
worn  (pp.  227-228).  They  may  consist  of  minerals  (quartz, 
chert,  felspars,  mica,  etc.),  or  of  rocks  (slate,  limestone, 
basalt,  etc.),  or  of  the  remains  of  plants  or  animals  (spores 
of  lycopods,  fragments  of  shells,  crinoids,  etc.).  It  is  evi- 
dent therefore  that  though  some  of  them  may  be  crystalline, 
the  rock  of  which  they  now  form  part  is  a  non-crystalline 
compound.  Water,  with  carbonate  of  lime  or  other  min- 
eral matter  in  solution,  permeating  a  detrital  rock,  has  some- 
times allowed  its  dissolved  materials  to  crystallize  among 
the  interstices  of  the  detritus,  thus  producing  a  more  or  less 
distinctly  crystalline  structure.  But  the  fundamentally  sec- 
ondary or  derivative  nature  of  the  mass  is  not  always  thereby 
effaced. 

2.  Microscopic  Structures  of  Rocks 

We  have  next  to  consider  the  manner  in  which  the  fore- 
going microscopic  elements  are  associated  in  rocks.  This 


GEOGNOSY  209 

inquiry  brings  before  us  the  minute  structure  or  texture  of 
rocks,  and  throws  great  light  upqn  their  origin  and  history.*8 

Four  types  of  rock-structure  are  revealed  by  the  micro- 
scope. A,  holocrystalline;  B,  hemi-crystalline;  C,  glassy; 
D,  clastic. 

A.  HOLOCRYSTALLINE,  consisting  entirely  of  crystals  or 
crystalline  individuals,  whether  visible  to  the  naked  eye, 
or  requiring  the  aid  of  a  microscope,  imbedded  in  each 
other  without  any  intervening  amorphous  substance.  Rocks 
of  this  type  are  exemplified  by  granite  (Figs.  15  and  29)  and 
by  other  igneous  rocks.  But  they  occur  also  among  the 
crystalline  limestones  and  schists,  as  in  statuary  marble, 
which  consists  entirely  of  crystalline  granules  of  calcite 
(Fig.  28). 

According  to  the  classification  proposed  by  Prof.  Rosen- 
busch  the  holocrystalline  structure  is  idiomorphic  or  panidio- 
morphic  when  each  of  the  component  crystals  has  assumed 
its  own  crystallographic  form,  and  allotriomorphic  when  it 
has  its  outlines  determined  by  those  of  its  neighbors.  When 
interspaces  have  been  left  between  the  crystals  or  crystal- 
line grains  the  structure  is  miarolitic  or  saccharoid. 

The  holocrystalline  eruptive  rocks  (p.  269)  are  typically 
represented  by  granite,  hence  the  term  granitoid  has  been 
adopted  to  express  their  microscopic  structure.  Varieties 
of  this  structure  are  designated  according  to  the  relations  ,of 
the  component  minerals.  Where  no  one  mineral  greatly 
preponderates,  but  where  they  are  all  confusedly  and  toler- 
ably equally  distributed  in  individuals  readily  observable 
by  the  naked  eye,  as  ordinary  granite,  the  structure  is 

98  The  first  broad  classification  of  the  microscopic  structure  of  rocks  was  that 
proposed  by  Zirkel,  which,  with  slight  modification,  is  here  adopted.  "Mik. 
Beschaff."  p.  265.  "Basaltgesteine, "  p.  88.  See  also  Rosenbusch's  suggestive 
paper  already  cited,  Neues.  Jahrb.  1882  (ii.),  p.  1. 


210 


TEXT-BOOK    OF   GEOLOGY 


granitic  (see  granular,  p.  177).  Where  a  similar  struc- 
ture is  so  fine  that  it  can  only  be  recognized  with  the 
microscope,  it  has  been  called  microgranitic  or  euritic. 
Where  the  minerals  are  grouped  in  small,  isolated,  grain- 
like  individuals,  each  having  its  own  independent  crystal- 
line structure,  so  that  under  the  microscope  in  polar- 
ized light,  the  rock  presents  the  appearance  of  a  brilliant 


Fig.    15  -Holocrvstalline   Structure.  Pig.  16.-Hemi-crystallme  Structure. 

Granite  (20  Diameters).    The  white  Do  erite,  consisting  ol  a  tnclimc 

portions  are  Quartz/the  striped  Felspar,  Augite    and  Magnetite 

parts  Felspar,  the  long,  dark,  finely  IP.  a  devitrihed  Ground-mass  (20 

striked   stripes   are   Mica.     (See  Wa^).^=ero^ 

the  broader  monoclinic  forms, 
slightly  shaded  in  the  drawing-, 
are  Augite;  the  black  specks  are 
Magnetite;  the  needle-shaped 
forms  are  Apatite.  (See  p.  294.) 

mosaic,  the  structure  has  been  named  granulitic  or  micro- 
granulitic  (panidiomorphic  granular  or  porphyric  of  Rosen  - 
busch).  Where  the  quartz  and  felspar  of  a  granitic  rock 
have  crystallized  together,  one  within  the  other,  the  struc- 
ture is  pegmatitic  (Fig.  31)  where  visible  to  the  naked  eye, 
and  micropegmatitic  (granophyric  of  Rosenbusch)  where  the 
help  of  a  microscope  is  needed  (Fig.  5).9' 


99  Fouque  and  Michel-Levy,  "Min.  Micrograph."  The  micropegmatite  of 
Michel-Levy  is  the  same  as  the  structure  subsequently  named  granophyre  by 
Rosenbusch.  Michel-Levy,  "Roches  Eruptives,"  p.  19. 


GEOGNOSY  211 

B.  HEMI-CRYSTALLINE.'"" — This  division  probably  com- 
prehends the  majority  of  the  massive  eruptive  or  igneous 
rocks.  It  is  distinguished  by  the  occurrence  of  what  ap- 
pears to  the  naked  eye  as  a  compact  or  finely  granular 
ground-mass,  through  which  more  or  less  recognizable  crys- 
tals are  scattered.  Examined  with  the  microscope,  this 
ground-mass  is  found  to  present  considerable  diversity 
(Figs.  16.  18,  32).  It  may  be  (1)  wholly  a  glass,  as  in  some 
basalts,  trachytes,  and  other  volcanic  products;  (2)  partly 
devitrified  through  separation  of  peculiar  little  granules 
and  needles  (crystallites  and  microlites)  which  appear  in 
a  vitreous  base;  (3)  still  further  devitrified,  until  it  becomes 
an  aggregation  of  such  little  granules,  needles,  and  hairs, 
between  which  little  or  no  glass-base  appears  (micro-crystal- 
litic);  or  (4)  "microfelsitic"  (petrosiliceous),  closely  related 
to  the  two  previous  groups,  and  consisting  of  a  nearly  struc- 
tureless mass,  marked  usually  with  indefinite  or  half -effaced 
granules  and  filaments,  but  behaving  like  a  singly-refracting, 
amorphous  body  (p.  204). 

In  rocks  belonging  to  this  type,  a  spherulitic  structure 
has  sometimes  been  produced  by  the  appearance  of  globular 
bodies  composed  of  a  crystalline  internally  radiating  sub- 
stance, sometimes  with  concentric  shells  of  amorphous 
material.  In  many  cases,  spherulites  are  only  recognizable 
with  the  microscope,  when  they  each  present  a  black  crgss 
between  crossed  Nicol-prisms,  and  thereby  characteristically 
reveal  the  microspherulitic  structure  (Figs.  7  and  17). 101 

100  por  tnja  structure  the  term  "mixed"  has  been  proposed,  as  being  a  mix- 
ture of  the  crystalline  and  amorphous  (glassy)  structures.     It  has  been  desig- 
nated by  Fouqu^  and  Michel-Levy  "trachytoid, "  as  being  typically  developed 
among  the  trachytes  (postea,  p.  288).    It  is  called  "hypocrystalline"  by  Rosen- 
busch. 

101  Fouque  and  Michel-Levy,  "Min.  Micrograph."     Some  remarkably  beau- 
tiful examples  of  microspherulitic  structure  occur  in  the  quartz- porphyries  that 
traverse  the  lower  Cambrian  tuffs  at  St.  David's.    Q.  J.  Geol.  Soc.  xxxix.  p.  313. 


212  TEXT-BOOK   OF   GEOLOGY 

The  term  op  hi  tic  is  applied  to  a  structure  in  which 
one  mineral  after  crystallizing  has  been  inclosed  within  an- 
other- during  the  consolidation  of  an  igneous  rock  (Fig.  18). 
It  is  abundant  in  many  dolerites  and  diabases  where  some 
bisilicate  such  as  augite  serves  as  a  matrix  in  which  the 
felspars  and  other  crystals  are  inclosed.  The  name  is  de- 
rived from  the  so-called  "ophites"  of  the  Pyrenees.102 


Fig.  17.— Spherulitic  Structure.    Pitch-  Fig.  18.— Ophitic  Structure.    Dolerite, 

stone,  Raasay  (magnified).  Skye  (magnified). 

C.  GLASSY. — Composed  of  a  volcanic  glass  such  as  has 
already  been  described.  It  seldom  happens,  however,  that 
rocks  which  seem  to  the  eye  to  be  tolerably  homogeneous 
glass  do  not  contain  abundant  crystallites  and  minute  crys- 
tals. Hence  truly  vitreous  rocks  tend  to  graduate  into  the 
second  or  hemi-crystalline  type.  This  gradation  and  the 
abundant  traces  of  a  devitrified  base  or  magma  between 
the  crystals  of  a  vast  number  of  eruptive  rocks,  lead  to  the 
belief  that  the  glassy  type  was  the  original  condition  of  most 
if  not  all  of  these  rocks.  Erupted  as  molten  masses,  their 
mobility  would  depend  upon  the  fluidity  of  the  glass.  Yet 
even  while  still  deep  within  the  earth's  crust,  some  of  their 
constituent  minerals  (felspars,  leucite,  magnetite,  etc.)  were 

102  These  rocks  (diabases)  have  been  critically  studied  by  J.  Kuhn,  Zeitsch. 
Deutsch.  Geol.  Ges.  xxxiii.  (1881)  372. 


GEOGNOSY  213 

often  already  crystallized,  and  suffered  fracture  and  corrosion 
by  subsequent  action  of  the  inclosing  magma.  This  is  well 
shown  by  what  is  termed  the  flow-structure  or  fluxion- 
structure.  Crystals  and  crystallites  are  ranged  in  current- 
like  lines,  with  their  long  axes  in  the  direction  of  these 
lines.  Where  a  large  older  crystal  occurs,  the  train  of 
minuter  individuals  is  found  to  sweep  round  it  and  to 
reunite  on  the  further  side,  or  to  be  diverted  in  an  eddy- 
like  course  (Fig.  19).  So  thoroughly  is  this  arrangement 
characteristic  of  the  motion  of  a  somewhat  viscid  liquid, 
that  there  cannot  be  any  doubt  that  such  was  the  condition 


Fig.  19.— Flow-structure  in  Obsidian.  ilg.  30.— Perlitic  Structure.    Felsitic 

(20  Diameters).  glass.    Mull  (magnified). 

of  these  masses  before  their  consolidation.  The  flow- 
structure  may  be  detected  in  many  eruptive  rocks,  from 
thoroughly  vitreous  compounds  like*  obsidian,  on  the  one 
hand,  to  completely  crystalline  masses  like  some  dolecites, 
on  the  other.  It  occurs  not  only  in  what  are  usually  re- 
garded as  volcanic  rocks,  but  also  in  plutonic  or  deep-seated 
masses  which,  there  is  reason  to  believe,  consolidated  be- 
neath the  surface,  as  for  instance^in  the  Bode  vein  of  the 
Harz,  among  quartz-porphyries  associated  with  granites  in 
Aberdeenshire,  and  in  felsite  dikes  and  bosses  in  the 
Shetlands,  Skye,  central  Scotland,  and  County  Waterford. 


214  TEXT-BOOK    OF   GEOLOGY 

The  structure,  therefore,  cannot  be  regarded  as  certainly 
indicating  that  the  rock  in  which  it  is  found  ever  flowed 
out  at  the  surface  as  lava. 

Some  glassy  rocks,  in  cooling  and  consolidating,  have 
had  spherulites  developed  in  them  (Fig.  17);  also  by  con- 
traction the  system  of  reticulated  and  spiral  cracks  known 
as  perlitic  structure  (p.  180  and  Figs.  9  and  20). 

The  final  stiffening  of  a  vitreous  mass  into  solid  stone 
has  resulted  (1st)  from  mere  solidification  of  the  glass:  this 
is  well  seen  at  the  edge  of  dikes  and  intrusive  sheets  of 
different  basalt-rocks,  where  the  igneous  mass,  having  been 
suddenly  congealed  along  its  line  of  contact  with  the  sur- 
rounding rocks,  remains  there  in  the  condition  of  glass, 
though  only  an  inch  further  inward  from  the  edge  the 
vitreous  magma  has  disappeared,  as  represented  in  Fig. 
287;  (2d)  from  the  devitrification  of  the  glass  by  the  abun- 
dant development  of  microfelsitic  granules  and  filaments, 
as  in  quartz-porphyry,  or  of  crystallites,  microlites  and 
crystals,  as  in  such  glassy  rocks  as  obsidian  and- tachy lite; 
or  (3d)  from  the  complete  crystallization  of  the  whole  of  the 
original  glassy  base,  as  may  be  observed  in  some  dolerites. 

D.  CLASTIC. — Composed  of  detrital  materials,  such  as 
have  been  already  described  (p.  183  and  Fig.  21).  Where 
these  materials  consist  of  grains  of  quartz-sand,  they  with- 
stand almost  any  subsequent  change,  and  hence  can  be 
recognized  even  among  a  highly  metamorphosed  series 
of  rocks.  Quartzite  from  such  a  series  can  sometimes  be 
scarcely  distinguished  under  the  microscope  from  unaltered 
quartzose  sandstone.  Where  the  detritus  has  resulted  from 
the  destruction  of  aluminous  or  magnesian  silicates,  it  is 
more  susceptible  of  alteration.  Hence  it  can  be  traced  in 
regions  of  local  metamorphism,  becoming  more  and  more 


GEOGNOSY 


215 


crystalline,  until  the  rocks  formed  of  or  containing  it  pass 
into  true  crystalline  schists. 

Detritus  derived  from  the  comminution  or  decay  of 
organic  remains  presents  very  different  and  characteristic 
structures103  (Fig.  22).  Sometimes  it  is  of  a  siliceous  nature, 
as  where  it  has  been  derived  from  diatoms  and  radiolarians. 
But  most  of  the  organically-derived  detrital  rocks  are  cal- 
careous, formed  from  the  remains  of  foraminifera,  corals, 
echinoderms,  polyzoa,  cirripeds,  annelids,  mollusks,  crusta- 
cea  and  other  invertebrates,  with  occasional  traces  of  fishes 


Fig.  21.  —  Clastic  Structure,  of  Inor- 
game  origin—  Section  of  a  Piece 
of  Greywacke.  (10  Diameters. 
See  p.  232.) 


Pig  22.  —  Clastic  Structure,  of  Organic 
Origin—  Structure  of  Chalk  (Sorby). 
Magnified  100  Diameters.  (See  p. 
2460 


or  even  of  higher  vertebrates.  Distinct  differences  of  micro- 
scopic structure  can  be  detected  in  the  hard  parts  of  some  of 
the  living  representatives  of  these  forms,  and  similar  differ- 
ences have  been  detected  in  beds  of  limestone  of  all  ages. 
Mr.  Sorby,  in  the  paper  cited  below,  has  shown  how  char- 
acteristic and  persistent  are  some  of  these  distinctions,  and 
how  they  may  be  made  to  indicate  the  origin  of  the  rock 
in  which  they  occur.  There  is  an  important  difference  be- 
tween the  two  forms  in  which  carbonate  of  lime  is  made 

103  ^6  gtudent  who  would  further  investigate  this  subject,  will  find  a  sug- 
gestive and  luminous  essay  upon  it  by  Mr.  Sorby  in  his  Presidential  Address  to 
the  Geological  Society,  Quart.  Journ.  Geol.  Soc.  1879. 


216  TEXT-BOOK   OF   GEOLOGY 

use  of  by  invertebrate  animals;  aragonite  being  much  less 
durable  than  calcite  (pp.  141,  244).  Hence  while  shells  of 
gasteropods,  many  lamellibranchs,  corals  and  other  organ- 
isms, formed  largely  or  wholly  of  aragonite,  crumble  down 
into  mere  amorphous  mud,  pass  into  crystalline  calcite,  or 
disappear,  the  fragments  of  those  consisting  of  calcite  may 
remain  quite  recognizable. 

It  is  evident,  therefore,  that  the  absence  of  all  trace  of 
organic  structure  in  a  limestone  need  not  invalidate  an  in- 
ference from  other  evidence  that  the  rock  has  been  formed 
from  the  remains  of  organisms.  The  calcareous  organic 
debris  of  a  sea-bottom  may  be  disintegrated,  and  reduced 
to  amorphous  detritus,  by  the  mechanical  action  of  waves 
and  currents,  by  the  solvent  chemical  action  of  the  water, 
by  the  decay  of  the  binding  material,  such  as  the  organic 
matter  of  shells,  or  by  being  swallowed  and  digested  by 
other  animals  (postea,  p.  243). 104 

Moreover,  in  clastic  calcareous  rocks,  owing  to  their 
liability  to  alteration  by  infiltrating  water,  there  is  a  ten- 
dency to  acquire  an  internal  crystalline  texture  (p.  621). 
At  the  time  of  formation,  little  empty  spaces  lie  between 
the  component  granules  and  fragments,  and  according  to 
Mr.  Sorby  these  interspaces  may  amount  to  about  a  quarter 
of  the  whole  mass  of  the  rock.  They  have  very  commonly 
been  filled  up  by  calcite  introduced  in  solution.  This  infil- 
trated calcite  acquires  a  crystalline  structure,  like  that  of 
ordinary  mineral- veins.  But  the  original  component  organic 
granules  also  themselves  become  crystalline,  and,  save  in 
so  far  as  their  external  contour  may  reveal  their  original 


JM  Sorby,  Presidential  Address,  Q.  J.  Geol.  Soc.  1879.  G.  Rose.  AbhandL 
Acad.  Berlin,  1858;  Giimbel,  Zeitsch.  Dentach.  Geol.  Gesellsch.  1884,  p.  38& 
Cornish  and  Kendall,  Geol.  Mag.  1888,  p.  66. 


GEOGNOSY  217 

organic  source,  they  cannot  be  distinguished  from  mere 
mineral-grains.  In  this  way,  a  cycle  of  geological  change 
is  completed.  The  calcium-carbonate  originally  dissolved 
out  of  rocks  by  infiltrating  water,  and  carried  into  the  sea, 
is  secreted  from  the  oceanic  waters  by  corals,  foraminifera, 
echinoderms,  mollusks  and  other  invertebrates.  The  re- 
mains of  these  creatures  collected  on  the  sea-bottom  slowly 
accumulate  into  beds  of  detritus,  which  in  after  times  are 
upheaved  into  land.  Water  once  more  percolating  through 
the  calcareous  mass,  gradually  imparts  to  it  a  crystalline 
structure,  and  eventually  all  trace  of  organic  forms  may 
be  effaced.  But  at  the  same  time,  the  rock,  once  exposed 
to  meteoric  influences,  is  attacked  by  carbonated  water,  its 
molecules  are  carried  in  solution  into  the  sea,  where  they 
will  again  be  built  up  into  the  framework  of  marine  or- 
ganisms. 

E.  ALTERATION  OF  ROCKS  BY  METEORIC  WATER. — An 
important  revelation  of  the  microscope  is  the  extent  to  which 
rocks  suffer  from  the  influence  of  infiltrating  water.  The 
nature  of  some  of  these  changes  is  described  in  subsequent 
pages.  (Book  III.  Part.  II.  Sect.  ii.  §  2.)  It  may  be  suffi- 
cient to  note  here  a  few  of  the  more  obvious  proofs  of  altera- 
tion. Threads  and  kernels  of  calcite  running  through  an 
eruptive  rock,  such  as  diabase,  dolerite,  or  andesite,  are  a 
good  index  of  internal  decomposition.  They  usually  pojnt 
to  the  decay  of  some  lime-bearing  mineral  in  the  rock. 
Some  other  minerals  are  likewise  frequent  signs  of  altera- 
tion, such  as  serpentine  (often  resulting  from  the  alteration 
of  olivine,  Figs.  33,  34),  chlorite,  epidote,  limonite,  chalced- 
ony, etc.  In  many  cases,  however,  the  decomposition  prod- 
ucts are  so  indefinite  in  form  and  so  minute  in  quantity  as 
not  to  permit  of  their  being  satisfactorily  referred  to  any 

GEOLOGY— Vol.  XXIX— 10 


218  TEXT-BOOK    OF   GEOLOGY 

known  species  of  mineral.  For  these  indeterminate,  but 
frequently  abundant  substances,  the  following  short  names 
were  proposed  by  Vogelsang  to  save  periphrasis,  until  the 
true  nature  of  the  substance  is  ascertained.  Viridite — green 
transparent  or  translucent  patches,  often  in  scaly  or  fibrous 
aggregations,  of  common  occurrence  in  more  or  less  de- 
composed rocks  containing  hornblende,  augite,  or  olivine: 
probably  in  many  cases  serpentine,  in  others  chlorite  'or 
delessite.  Ferrite — yellowish,  reddish,  or  brownish  amor- 
phous substances,  probably  consisting  of  peroxide  of  iron, 
either  hydrous  or  anhydrous,  but  not  certainly  referable  to 
any  mineral,  though  sometimes  pseudomorphous  after  ferru- 
ginous minerals.  Opacite — black,  opaque  grains  and  scales 
of  amorphous  earthy  matter,  which  may  in  different  cases  be 
magnetite,  or  some  other  metallic  oxide,  earthy  silicates, 
graphite,  etc.105 

§  vi.  Classification  of  Rocks 

It  is  evident  that  Lithology  may  be  approached  from  two 
very  different  sides.  We  may,  on  the  one  hand,  regard 
rocks  chiefly  as  so  many  masses  of  mineral  matter,  present- 
ing great  variety  of  chemical  composition  and  marvellous 
diversity  of  microscopic  structure.  Or,  on  the  other  hand, 
passing  from  the  details  of  their  chemical  and  mineralogical 
characters,  we  may  look  at  them  rather  as  the  records  of 
ancient  terrestrial  changes.  In  the  former  aspect,  they  pre- 
sent for  consideration  problems  of  the  highest  interest  in  in- 
organic chemistry  and  mineralogy;  in  the  latter  view,  they 
invite  attention  to  the  great  geological  revolutions  through 
which  the  planet  has  passed.  It  is  evident,  therefore,  that 


106  Vogelsang,  Z.  Deutsch.  Geol.  Ges.  xxiv.  (1872),  p.   529.     Zirkel,  Geol. 
Expl.  40th  Parallel,  vol.  vi.  p.  12. 


GEOGNOSY  219 

two  distinct  systems  of  classification  may  be  followed,  the 
one  based  on  chemical  and  mineralogical,  the  other  on 
geological  considerations. 

From  a  chemical  point  of  view,  rocks  may  be  grouped 
according  to  their  composition;  as  Oxides,  exemplified  by 
formations  of  quartz,  hsematite,  or  magnetite;  Carbonates, 
including  the  limestones  and  clay-ironstones;  Silicates,  em- 
bracing the  vast  majority  of  rocks,  whether  composed  of  a 
single  mineral,  or  of  more  than  one;  Phosphates,  such  as 
guano  and  the  older  bone- beds  and  coprolitic  deposits.  A 
classification  of  this  kind,  however,  pays  no  regard  to  the 
mode  of  origin  or  conditions  of  occurrence  of  the  rocks,  and 
is  not  well  suited  for  the  purposes  of  the  geologist.106 

From  the  mineralogical  side,  rocks  may  be  classified  with 
reference  to  their  prevailing  mineral  constituent.  Thus  such 
subdivisions  as  Calcareous  rocks,  Quartzose  rocks,  Ortho- 
clase  rocks,  Plagioclase  rocks,  Pyroxenic  rocks,  Horn- 
blendic  rocks,  etc.,  may  be  adopted;  but  these  terms  are 
hardly  less  objectionable  to  the  geologist,  and  are  in  fact 
suited  rather  for  the  arrangement  of  hand-specimens  in  a 
museum,  than  for  the  investigation  of  rocks  in  situ. 

From  the  standpoint  of  geological  inquiry,  rocks  have 
been  classified  according  to  their  mode  of  origin.  In  one 
system  they  are  arranged  under  three  great  divisions:  1st, 
Igneous,  embracing  all  which  have  been  erupted  from  the 
heated  interior  of  the  earth;  2d,  Aqueous  or  Sedimentary, 
including  all  which  have  been  laid  down  as  mechanical  or 
chemical  deposits  from  water  or  air,  and  all  which  have 
resulted  from  the  growth  and  decay  of  plants  or  animals; 
3d,  MetamorphiCj  those  which  have  undergone  subsequent 

106  The  eruptive  rocks  are  susceptible  of  a  convenient,  though  not  strictly 
accurate,  chemical  classification  into  acid,  intermediate  and  basic  (see  p.  273). 


220  TEXT-BOOK    OF    GEOLOGY 

change  within  the  crust  of  the  earth,  whereby  their  original 
character  has  been  so  modified  as  to  be  sometimes  quite 
indeterminable.  Another  geological  arrangement  is  based 
upon  the  general  structure  of  the  rocks,  and  consists  of  two 
divisions:  1st,  Stratified,  embracing  all  the  aqueous  and  sedi- 
mentary, with  part  of  the  less  altered  metamorphic  rocks; 
2d,  Unstratified,  nearly  conterminous  with  the  term  igne- 
ous, since  it  includes  all  the  eruptive  rocks.  Further  sub- 
divisions of  this  series  have  been  proposed  according  to  dif- 
ferences of  structure  or  texture,  as  porphyritic,  granitic,  etc. 
These  geological  subdivisions,  however,  ignore  the  chemical 
and  mineralogical  characters  of  the  rocks,  and  are  based  on 
deductions  which  may  not  always  be  sound.  Thus,  rocks 
may  be  included  in  the  igneous  series  which  further  re- 
search may  show  not  to  be  of  igneous  origin;  others  may  be 
classed  as  metamorphic,  regarding  the  true  origin  of  which 
there  may  be  considerable  uncertainty. 

A  further  system  of  classification,  based  upon  relative 
age,  has  been  applied  to  the  arrangement  of  the  eruptive 
rocks,  those  masses  which  were  erupted  prior  to  Secondary 
time  being  classed  as  "older,"  and  those  of  Tertiary  and 
later  date  as  "younger. "  This  system  has  been  elaborated 
in  great  detail  by  Michel-Levy,  who  maintains  that  the  same 
types  have  been  reproduced  nearly  in  the  same  order  in  the 
two  series,  though  basic  rocks,  often  with  vitreous  charac- 
ters, rather  predominate  in  the  later.107  It  must,  indeed,  be 

1OT  See  on  this  subject,  J.  D.  Dana,  Amer.  J.  Sci.  xvi.  1878,  p.  336.  Michel- 
Levy,  Bull.  Soc.  Geol.  France,  3d  ser.  iii.  (1874),  p.  199;  vi.  p.  173.  Ann.  des 
Mines,  viii.  (1875)  "Roches  Eruptives,"  1889.  Fouque  and  Michel-Levy,  "Min- 
eralogie  Microgr."  p,  150.  Rosenbusch,  "Mik.  Physiog."  ii.  Reyer,  "Physik 
der  Eruptioaen,"  1877,  part  iii.  opposes  the  adoption  of  relative  age  as  a  basis 
of  classification.  On  the  classification  of  compound  silicated  rocks,  see  Vogel- 
sang, Z.  Deutsch.  Geol.  Ges.  xxiv.  p.  507 ;  and  for  an  incisive  criticism  of  a  too 
merely  mineralogical  classification.  Lessen,  op.  cit.  xxiv.  p.  782.  Consult  also 
0.  Lang,  "Ueber  die  Individual! tat  der  Gesteine"  in  Tschermak's  Min.  Miuheil. 
vol.  xi.  part  6  (1890),  p.  467. 


GEOGNOSY  221 

admitted  that  certain  broad  distinctions  between  the  older 
and  the  later  eruptive  rocks  have  been  well  ascertained,  and 
appear  to  hold  generally  over  the  world.  Among  these  dis- 
tinctions may  be  mentioned  as  more  characteristic  of  the 
Palaeozoic  rocks  the  presence  of  microcline,  turbid  ortho- 
clase  in  Carlsbad  twins,  muscovite,  enstatite,  bronzite,  dial- 
lage,  tourmaline,  anatase,  rutile,  cordierite,  and  in  the 
younger  rocks  the  presence  of  sanidine,  tridymite,  leucite, 
nosean,  hauyne,  and  zeolites.  Even  where  the  same  min- 
eral occurs  in  both  the  older  and  newer  series,  it  often  pre- 
sents a  somewhat  different  aspect  in  each,  as  in  the  case  of 
the  plagioclase  and  augite,  which  in  the  younger  series  are 
distinguished  by  the  occurrence  in  them  of  vitreous  and 
gaseous  inclusions  which  are  rare  or  absent  in  those  of  the 
older  series.108  Throughout  the  younger  eruptive  rocks,  the 
vitreous  condition  is  much  more  frequent  and  perfectly  de- 
veloped than  in  the  older  group,  where,  on  the  other  hand, 
the  granitic  structure  is  characteristically  displayed.  Still, 
to  these  rules  so  many  exceptions  occur  that  it  may  be 
doubted  whether  enough  of  positively  ascertained  data  have 
been  collected  regarding  the  relative  ages  of  eruptive  rocks 
to  warrant  the  adoption  of  any  classification  upon  a  chrono- 
logical basis.  There  can  be  no  doubt  that,  making  due 
allowance  for  the  alterations  arising  from  permeation  by 
meteoric  water,  there  is  no  essential  difference  between  some 
types  of  volcanic  rock  in  Paleozoic  and  in  recent  times. 
The  Carboniferous  basalts  and  trachytes  of  Scotland,  for 
example,  present  the  closest  resemblance  to  those  of  Ter- 
tiary age.109 

Though  no  classification   which  can  at  present  be  pro- 

108  See  J.  Murray  and  A.  Renard,  Proc.  Roy.  Soc.  Edin.  xi.  p.  669. 

109  See  Nature,  iii.  (1871),  p.  303. 


222  TEXT-BOOK    OF   GEOLOGY 

posed  is  wholly  satisfactory,  one  which  shall  do  least  vio- 
lence, at  once  to  geological  and  mineralogical  relationships, 
is  to  be  preferred.  The  arrangement  which  has  met  with 
the  most  general  acceptance  is  threefold.  1st,  Sedimen- 
tary Bocks,  including  first  the  rocks  which  have  resulted 
from  the  accumulation  of  detritus,  either  inorganic  or  or- 
ganic, under  water  or  on  land,  and  secondly  those  which 
have  been  deposited  from  aqueous  solution.  The  former 
are  mechanical,  the  latter  chemical  accumulations;  but  they 
have  often  been  deposited  together.  Certain  rocks  of  me- 
chanical origin,  such  as  detrital  limestones,  may  by  subse- 
quent alteration  be  converted  into  materials  that  cannot  be 
distinguished  from  others  of  true  chemical  origin.  Hence 
the  whole  series  is  intimately  linked  together.  2d,  Mas- 
sive, Eruptive,  or  Intrusive  Rocks,  embracing  all 
those  which  have  solidified  from  fusion  within  the  earth's 
crust,  or  have  been  erupted  as  lava  to  the  surface.  3d, 
Schistose  Rocks,  and  their  accompaniments,  including 
the  so-called  Metamorphic  rocks  which  have  reached  their 
present  condition  as  a  consequence  of  the  alteration  some- 
times of  sedimentary,  sometimes  of  igneous  rocks.  This 
group  graduates  into  the  two  others,  but  it  contains  some 
distinctive  masses,  the  origin  of  which  is  still  involved  in 
doubt. 

It  must  be  kept  in  view  that  in  this  proposed  system 
of  classification,  and  in  the  following  detailed  description  of 
rocks,  many  questions  regarding  the  origin  and  decomposi- 
tion of  these  mineral  masses  must  necessarily  be  alluded  to. 
The  student,  however,  will  find  these  questions  discussed  in 
later  pages,  and  will  probably  recognize  a  distinct  advantage 
in  this  unavoidable  preliminary  reference  to  them  in  connec- 
tion with  the  rocks  by  which  they  are  suggested. 


GEOGNOSY  M6 

§  vii.  A  Description  of  .the  more  Important  Rocks  of  the  Earth's  Crust 

Full  details  regarding  the  composition,  microscopic  struc- 
ture, and  other  characters  of  rocks  must  be  sought  in  such 
general  treatises  and  special  memoirs  as  those  already  cited 
(pp.  160,  172,  193).  The  purposes  of  the  present  text-book 
will  be  served  by  a  succinct  account  of  the  more  common  or 
important  rocks  which  enter  into  the  composition  of  the 
crust  of  the  earth. 

I.  SEDIMENTARY 

A.  FRAGMENTAL  (CLASTIC) 

This  great  series  embraces  all  rocks  of  a  secondary  or 
derivative  origin ;  in  other  words,  all  formed  of  fragmentary 
materials  which  have  previously  existed  on  or  beneath  the 
surface  of  the  earth  in  another  form,  and  the  accumulation 
and  consolidation  of  which  gives  rise  to  new  compounds. 
Some  of  these  materials  have  been  produced  by  the  me- 
chanical action  of  wind,  as  in  the  sand-hills  of  sea-coasts 
and  inland  deserts  (^Eolian  rocks);  others  by  the  operation 
of  moving  water,  as  the  gravel,  sand  and  mud  of  shores  and 
river-beds  (Aqueous  sedimentary  rocks);  others  by  the  ac- 
cumulation of  the  entire  or  fragmentary  remains  of  once  liv- 
ing plants  and  animals  (Organically -formed  rocks);  while 
yet  another  series  has  arisen  from  the  gathering  together  of 
the  loose  debris  thrown  out  by  volcanoes  (Volcanic  tuffs). 
It  is  evident  that  in  dealing  with  these  various  detrital  for- 
mations, the  degree  of  consolidation  is  of  secondary  impor- 
tance. The  soft  sand  and  mud  of  a  modern  lake-bottom 
differ  in  no  essential  respect  from  ancient  lacustrine  strata, 
and  may  tell  their  geological  story  equally  well.  No  line  is 
to  be  drawn  between  what  is  popularly  termed  rock  and  the 


224  TEXT-BOOK    OF    GEOLOGY 

loose,  as  yet  uncompacted,  debris  out  of  which  solid  rocks 
may  eventually  be  formed.  Hence  in  the  following  arrange- 
ment, the  modern  and  the  ancient,  being  one  in  structure 
and  mode  of  formation,  are  classed  together. 

It  will  be  observed  that,  in  several  directions,  we  are  led 
by  the  fragmental  rocks  to  crystalline  stratified  deposits, 
some  of  which  have  been  deposited  from  chemical  solution, 
while  others  have  resulted  from  the  gradual  conversion  of 
a  detrital  into  a  crystalline  structure.  Both  series  of  de- 
posits are  accumulated  simultaneously  and  are  often  inter- 
stratified.  Calcareous  rocks  formed  of  organic  remains 
(p.  243)  exhibit  very  clearly  this  gradual  internal  change, 
which  more  or  less  effaces  their  detrital  origin,  and  gives 
them  such  a  crystalline  character  as  to  entitle  them  to  be 
ranked  among  the  crystalline  limestones. 

1.  Gravel  and  Sand  Rocks  (Psammites) 

As  the  deposits  included  in  this  subdivision  are  pro- 
duced by  the  disintegration  and  removal  of  rocks  by  the 
action  of  the  atmosphere,  rain,  rivers,  frost,  the  sea,  and 
other  superficial  agencies,  they  are  mere  mechanical  accu- 
mulations, and  necessarily  vary  indefinitely  in  composition, 
according  to  the  nature  of  the  sources  from  which  they  are 
derived.  As  a  rule,  they  consist  of  the  detritus  of  siliceous 
rocks,  these  being  among  the  most  durable  materials. 
Quartz,  in  particular,  enters  largely  into  the  composition  of 
sandy  and  gravelly  detritus.  Fragmentary  materials  tend 
to  group  themselves  according  to  their  size  and  relative 
density.  Hence  they  are  apt  to  occur  in  layers,  and  to 
show  the  characteristic  stratified  arrangement  of  sedimen- 
tary rocks.  They  may  inclose  the  remains  of  any  plants  or 
animals  entombed  on  the  same  sea-floor,  river-bed,  or  lake- 
bottom. 

In  the  majority  of  these  rocks,  their  general  mineral 
composition  is  obvious  to  the  naked  eye.  But  the  appli- 
cation of  the  microscope  to  their  investigation  has  thrown 
considerable  light  upon  their  composition,  formation,  and 
subsequent  mutations.  Their  component  materials  are  thus 


GEOGNOSY  225 

ascertained  to  be  divisible  into — 1st,  derived  fragments,  of 
which  the  most  abundant  are  quartz,  after  which  come  fel- 
spar, mica,  iron-ores,  zircon,  rutile,  apatite,  tourmaline, 
garnet,  sphene,  augite,  hornblende,  fragments  of  various 
rocks,  and  clastic  dust;  2d,  constituents  which  have  been 
deposited  between  the  particles,  and  which  in  many  cases 
serve  as  the  cementing  material  of  the  rock.  Among  the 
more  important  of  these  are  silicic  acid  in  the  form  of 
quartz,  chalcedony  and  opal;  carbonates  of  lime,  iron  or 
magnesia;  haematite,  limonite;  pyrite  and  glauconite.110 

Cliff-Debris,  Moraine  StdF — angular  rubbish  disengaged  by 
frost  and  ordinary  atmospheric  waste  from  cliffs,  crags,  and 
steep  slopes.  It  slides  down  the  declivities  of  hilly  regions, 
and  accumulates  at  their  base,  until  washed  away  by  rain 
or  by  brooks.  It  forms  talus-slopes  of  as  much  as  40°, 
though  for  short  distances,  if  the  blocks  are  large,  the 
general  angle  of  slope  may  be  much  steeper.  It  naturally 
depends  for  its  composition  upon  the  nature  of  the  solid 
rocks  from  which  it  is  derived.  Where  cliff-debris  falls 
upon  and  is  borne  along  by  glaciers  it  is  called  "Moraine- 
stuff,"  which  may  be  deposited  near  its  source,  or  may  be 
transported  for  many  miles  on  the  surface  of  the  ice  (p.  714). 

Perched  Blocks,  Erratic  Blocks — large  masses  of  rock,  often  as 
big  as  a  house,  which  have  been  transported  by  glacier-ice, 
and  have  been  lodged  in  a  prominent  position  in  glacier 
valleys  or  have  been  scattered  over  hills  and  plains.  An 
examination  of  their  mineralogical  character  leads  to  the 
identification  of  their  source,  and,  consequently,  to  the 

Stth  taken  by  the  transporting  ice.  (See  liook  III.  Part 
.  Section  ii.  §  5.) 

Rain-wash — a  loam  or  earth  which  accumulates  on  the 
lower  parts  of  slopes  or  at  their  base,  and  is  due  to 
the  gradual  descent  of  the  finer  particles  of  disintegrated 
rocks  by  the  transporting  action  of  rain.  Brick-earth 
is  the  name  given  in  the  southeast  of  England  to  thick 
masses  of  such  loam,  which  is  extensively  used  for  making 
bricks. 

Soil— the  product  of  the  subaerial  decomposition  of  rocks 
and  of  the  decay  of  plants  and  animals.  Primarily  the 
character  of  the  soil  is  determined  by  that  of  the  subsoil, 
of  which  indeed  it  is  merely  a  further  disintegration.  Ac- 

110  G.  Klemm,  Zeitsch.  Deutsch.  Geol.  Ges.  xxxiv.  (1882),  p.  771.  H.  C. 
Sorby,  Quart.  Journ.  Geol.  Soc.  xxxvi.  (1880).  J.  A.  Phillips,  op.  cit.  xxxvii. 
(1881),  p.  6. 


226  TEXT-BOOK   OF   GEOLOGY 

cording  to  the  nature  of  the  rock  underneath,  a  soil  may 
vary  from  a  stiff  clay,  through  various  clayey  and  sandy 
loams,  to  mere  sand.  The  formation  of  soil  is  treated  of 
in  Book  III.  Part  II.  Section  ii.  §  1. 

Subsoil — the  broken- up  part  of  the  rocks  immediately 
under  the  soil.  Its  character,  of  course,  is  determined  by 
that  of  the  rock  out  of  which  it  is  formed  by  subaerial  dis- 
integration. (Book  III.  Part  II.  Section  ii.  §  1.) 

Blown  Sand — loose  sand  usually  arranged  in  lines  of 
dunes,  fronting  a  sandy  beach  or  in  the  arid  interior  of 
a  continent.  It  is  piled  up  by  the  driving  action  of  wind. 
(Book  III.  Part  II.  Section  i.)  It  varies  in  composition, 
being  sometimes  entirely  siliceous,  as  upon  shores  where 
siliceous  rocks  are  exposed;  sometimes  calcareous,  where 
derived  from  triturated  shells,  nullipores,  or  other  calca- 
reous organisms.  The  minute  grains  from  long-continued 
mutual  friction  assume  remarkably  rounded  and  polished 
forms.  Layers  of  finer  and  coarser  particles  often  alternate, 
as  in  water-formed  sandstone.  On  many  coast-lines  in 
Europe,  grasses  and  other  plants  bind  the  surface  of  the 
shifting  sand.  These  layers  of  vegetation  are  apt  to  be 
covered  by  fresh  encroachments  of  the  loose  material,  and 
then  by  their  decay  to  give  rise  to  dark  peaty  seams  in  the 
sand.  Calcareous  blown  sand  is  compacted  into  hard  stone 
by  the  action  of  rain-water,  which  alternately  dissolves  a 
little  of  the  lime,  and  re-deposits  it  on  evaporation  as  a  thin 
crust  cementing  the  grains  of  sand  together.  In  the  Baha- 
mas and  Bermudas,  extensive  masses  of  calcareous  blown 
sand  have  been  cemented  in  this  way  into  solid  stone,  which 
weathers  into  picturesque  crags  and  caves  like  a  limestone 
of  older  geological  date.111  At  Newquay,  Cornwall,  blown 
sand  has  been  by  the  decay  of  abundant  land-shells  solidi- 
fied into  a  material  capable  of  being  used  as  a  building- 
stone. 

River-sand,  Sea-sand. — When  the  rounded  water-worn  de- 
tritus is  finer  than  that  to  which  the  term  gravel  would  be 
applied,  it  is  called  sand,  though  there  is  obviously  no  line 
to  pe  drawn  between  the  two  kinds  of  deposit,  which  neces- 
sarily graduate  into  each  other.  The  particles  of  sand  range 
down  to  such  minute  forms  as  can  only  be  distinctly  dis- 

111  For  interesting  accounts  of  the  ./Eolian  deposits  of  the  Bahama  and  Ber- 
mudas, see  Nelson,  Q.  J.  Geol.  Soc.  ix.  p.  200,  Sir  Wyville  Thomson's  "Atlan- 
tic," vol.  i. ;  also  J.  J.  Rein,  Senckenb.  Nat.  Gesellsch.  Bericht.  1869-70,  p.  140, 
1872-73,  p.  131.  On  the  Red  Sands  of  the  Arabian  Desert,  see  J.  A.  Phillips, 
Q.  J.  Geol.  Soc.  xxxviii.  (1882),  p.  110,  also  op.  cit.  xxzvii.  (1881),  p.  12. 


GEOGNOSY  227 

cerned  with  a  microscope.  The  smaller  forms  are  generally 
less  well  rounded  than  those  of  greater  dimensions,  no  doubt 
because  their  diminutive  size  allows  them  to  remain  sus- 
pended in  agitated  water,  and  thus  to  escape  the  mutual 
attrition  to  which  the  larger  and  heavier  grains  are  exposed 
upon  the  bottom.  (Book  III.  Part  II.  Section  ii.J  So  far 
as  experience  has  yet  gone,  there  is  no  method  by  which 
inorganic  sea-sand  can  be  distinguished  from  that  of  rivers 
or  lakes.  As  a  rule,  sand  consists  largely  (often  wholly) 
of  quartz-grains.  The  presence  of  fragments  of  marine 
shells  will  of  course  betray  its  salt-water  origin;  but  in 
the  trituration  to  which  sand  is  exposed  on  a  coast-line, 
the  shell-fragments  are  in  great  measure  ground  into  cal- 
careous mud  and  removed. 

Mr.  Sorby  has  shown  that,  by  microscopic  investigation, 
much  information  may  be  obtained  regarding  the  history 
and  source  of  sedimentary  materials.  He  has  studied  the 
minute  structure  of  modern  sand,  and  finds  that  sand-grains 
present  the  following  five  distinct  types,  which,  however, 
graduate  into  each  other. 

1.  Normal,  angular,  fresh -formed  sand,  such  as  has  been 
derived  almost  directly  from  the  breaking  up  of  granitic  or 
schistose  rocks. 

2.  Well-worn  sand  in  rounded  grains,  the  original  angles 
being  completely  lost,   and  the  surfaces  looking  like  fine 
ground  glass. 

3.  Sand  mechanically  broken  into  sharp  angular  chips, 
showing  a  glassy  fracture. 

4.  Sand   having  the  grains  chemically  corroded,   so  as 
to  produce  a  peculiar  texture  of  the  surface,  differing  from 
that  of  worn  grains  or  crystals. 

5.  Sand  in  which  the  grains  have  a  perfectly  crystalline 
outline,  in  some  cases  undoubtedly  due   to  the  deposition 
of  quartz  upon  rounded  or  angular  nuclei  of  ordinary  non- 
crystalline  sand.11" 

The  same  acute  observer  points  out  that,  as  in  the 
familiar  case  of  conglomerate  pebbles,  which  have  some- 
times been  used  over  again  in  conglomerates  of  very  differ- 
ent ages,  so  with  the  much  more  minute  grains  of  sand,  we 
must  distinguish  between  the  age  of  the  grains  and  the  age 
of  the  deposit  formed  of  them.  An  ancient  sandstone  may 
consist  of  grains  that  had  hardly  been  worn  before  they  were 

u'2  Address,  Q.  J.  Geol.  Soc.  xxxvi.  (1880),  p.  58,  and  Monthly  Microscop. 
Journ.  Anniv.  Address,  1877. 


228  TEXT-BOOK    OF   GEOLOGY 

finally  brought  to  rest,  while  the  sand  of  a  modern  beach 
may  have  been  ground  down  by  the  vaves  of  many  suc- 
cessive geological  periods. 

Sand  taken  by  Mr.  Sorby  from  the  old  gravel  terraces 
of  the  River  Tay  was  found  to  be  almost  wholly  angular, 
indicating  how  little  wear  and  tear  there  may  be  among 
particles  of  quartz  TTO  of  an  inch  in  diameter,  even  though 
exposed  to  the  drifting  action  of  a  rapid  river.113  Sand  from 
the  bowlder  clay  at  Scarborough  was  likewise  ascertained  to 
be  almost  entirely  fresh  and  angular.  On  the  other  hand, 
in  geological  formations  which  can  be  traced  in  a  given 
direction  for  several  hundred  miles,  a  progressively  large 
proportion  of  rounded  particles  may  be  detected  in  the 
sandy  beds,  as  Mr.  Sorby  has  found  in  following  the  Green- 
sand  from  Devonshire  to  Kent.  In  wind-blown  sand  ex- 
posed for  a  long  period  to  drift  to  and  fro  along  the  sur- 
lace  the  larger  particles  and  pebbles  acquire  a  remarkably 
smoothed  and  polished  surface. 

The  occurrence  of  various  other  minerals  besides  quartz 
in  ordinary  sand  has  long  been  recognized,  but  we  owe  to 
the  recent  observations  of  Mr.  A.  B.  Dick  the  discovery 
that  among  these  minerals  some  of  the  most  plentiful  and 
most  perfectly  preserved  belong  to  species  that  were  not 
supposed  to  be  so  widely  diffused,  such  as  zircon,  rutile, 
and  tourmaline.  He  has  found  that  these  heavy  minerals 
constitute  sometimes  as  much  as  4  per  cent  of  the  Bagshot 
sand  of  the  older  Tertiary  series  of  the  London  basin.114 
Felspars,  micas,  hornblendes,  pyroxenes,  magnetite,  glau- 
conite  and  other  minerals  may  likewise  be  recognized.  The 
remarkable  perfection  of  some  of  the  crystallographic  forms 
of  the  minuter  mineral  constituents  of  certain  sands  has  been 
well  shown  by  Mr.  Dick. 

Varieties  of  river-  or  sea-sand  may  be  distinguished  by 
names  referring  to  some  remarkable 'constituent,  e.g.  mag- 
netic sand,  iron-sand,  gold-sand,  auriferous  sand,  etc. 

Gravel,  Shingle — names  applied  to  the  coarser  kinds  of 
rounded  water-worn  detritus.  In  Gravel,  the  average  size 
of  the  component  pebbles  ranges  from  that  of  a  small  pea  up 
to  about  that  of  a  walnut,  though  of  course  many  included 
fragments  will  be  observed  which  exceed  these  limits.  In 
Shingle,  the  stones  are  coarser,  ranging  up  to  blocks  as  big 
. —  _ 

113  See  Book  III.  Part  II.  Section  ii.  §  3. 

114  Nature,  xxxvi.   (1887),   p.  91,  Mem.  Geol.  Surv.  "Geology  of  London," 
vol.  i.  (1889),  p.  523.     Teall,  "Microscopic  Petrography,"  Plate  xliv. 


GEOGNOSY  229 

as  a  man's  head  or  larger.  German  geologists  distinguish 
as  "schotter,"  a  shingle  containing  dispersed  bowlders,  and 
"schotter-conglomerate,"  a  rock  wherein  these  materials 
have  become  consolidated.115  All  these  names  are  applied 
quite  irrespective  of  the  composition  of  the  fragments,  which 
varies  greatly  from  point  to  point.  As  a  rule,  the  stones 
consist  of  hard  rocks,  since  these  are  best  fitted  to  with- 
stand the  powerful  grinding  action  to  which  they  are 


Conglomerate  (PuddingstoneV— a  rock  formed  of  consoli- 
dated gravel  or  shingle.  The  component  pebbles  are 
rounded  and  water-worn.  They  may  consist  of  any  kind 
of  rock,  though  usually  of  some  hard  and  durable  sort, 
such  as  quartz  or  quartzite.  A  special  name  may  be  given 
according  to  the  nature  of  the  pebbles,  as  quartz-conglom- 
erate, limestone-conglomerate,  granite-conglomerate,  etc.,  or 
according  to  that  of  the  paste  or  cementing  matrix,  which 
may  consist  of  a  hardened  sand  or  clay,  and  may  be  sili- 
ceous, calcareous,  argillaceous,  or  ferruginous.  In  the  coarser 
conglomerates,  where  the  blocks  may  exceed  six  feet  in 
length,  there  is  often  very  little  indication  of  stratification. 
Except  where  the  flatter  stones  show  by  their  general  paral- 
lelism the  rude  lines  of  deposit,  it  may  be  only  when  the 
mass  of  conglomerate  is  taken  as  a  whole,  in  its  relation  to 
the  rocks  below  and  above  it,  that  its  claim  to  be  considered 
a  bedded  rock  will  be  conceded.  The  occurrence  of  occa- 
sional bands  of  conglomerate  in  a  series  of  arenaceous  strata 
is  analogous  probably  to  that  of  a  shingle-bank  or  gravel- 
beach  on  a  modern  coast-line.  But  it  is  not  easy  to  under- 
stand the  circumstances  under  which  some  ancient  conglom- 
erates accumulated,  such  as  that  of  the  Old  Red  Sandstone 
of  Central  Scotland,  which  attains  a  thickness  of  many  thou- 
sand feet,  and  consists  of  well-rounded  and  smoothed  blocks 
often  several  feet  in  diameter. 

In  many  old  conglomerates  (and  even  in  those  of  Miocene 
age  in  Switzerland)  the  component  pebbles  may  be  observed 
to  have  indented  each  other.  In  such  cases  also  they  may 
be  found  elongated,  distorted  or  split  and  recementect; 
sometimes  the  same  pebble  has  been  crushed  into  a  num- 
ber of  pieces,  which  are  held  together  by  a  retaining  cement. 
These  phenomena  point  to  great  pressure,  and  some  internal 


115  See,  for  example,  an  account  of  the  schotter-conglomerates  of  Northern 
Persia  by  E.  Tietze,  Jahrb.  Geol.  ReichsansU  Vienna,  188],  p.  68. 


230     -  TEXT-BOOK    OF   GEOLOGY 

relative  movement  in  the  rocks.  (Book  III.  Part  I.  Section 
iv.  §  3.) 

Breccia — a  rock  composed  of  angular,  instead  of  rounded, 
fragments.  It  commonly  presents  less  trace  of  stratification 
than  conglomerate.  Intermediate  stages  between  these  two 
rocks,  where  the  stones  are  partly  angular  and  partly  sub- 
angular  and  rounded,  are  known  as  brecciated  conglomerate. 
Considered  as  a  detrital  deposit  formed  by  superficial  waste, 
breccia  points  to  the  disintegration  of  rocks  by  the  atmos- 
phere, and  the  accumulation  of  their  fragments  with  little  or 
no  intervention  of  running  water.  Thus  it  may  be  formed 
at  the  base  of  a  cliff,  either  subaerially,  or  where  the  ddbris 
of  the  cliff  falls  at  once  into  a  lake  or  into  deep  sea-water. 

The  term  Breccia  has,  however,  been  applied  to  rocks 
formed  in  a  totally  different  manner.  -Angular  blocks  of 
all  sizes  and  shapes  have  been  discharged  from  volcanic 
orifices,  and,  falling  back,  have  consolidated  there  into 
masses  of  brecciated  material  (volcanic  breccia).  Intrusive 
igneous  eruptions  have  sometimes  torn  off  fragments  of 
the  rocks  through  which  they  have  ascended,  and  these 
angular  fragments  have  been  inclosed  in  the  liquid  or  pasty 
mass.  Or  the  intrusive  rock  has  cooled  and  solidified  ex- 
ternally while  still  mobile  within,  and  in  its  ascent  has 
caught  up  and  involved  some  of  these  consolidated  parts 
of  its  own  substance.  Again,  where  solid  masses  of  rock 
within  the  crust  of  the  earth  have  ground  against  each 
other,  as  in  dislocations,  angular  fragmentary  rubbish  has 
been  produced,  which  has  subsequently  been  consolidated 
by  some  infiltrating  cement  (Fault-rock).  It  is  evident, 
however,  that  breccia  formed  in  one  or  other  of  these 
hypogene  ways  will  not,  as  a  rule,  be  apt  to  be  mistaken 
for  the  true  breccias,  arising  from  superficial  disintegration. 

Sandstone  (Gres)116 — a  rock  composed  of  consolidated  sand. 
As  in  ordinary  modern  sand,  the  integral  grains  of  sandstone 
are  chiefly  quartz,  which  must  here  be  regarded  as  the  resi- 
due left  after  all  the  less  durable  minerals  of  the  original 
rocks  have  been  carried  away  in  solution  or  in  suspension  as 
fine  mud.  The  colors  of  sandstones  arise,  not  so  much  from 
that  of  the  quartz,  which  is  commonly  white  or  gray,  as 
from  the  film  or  crust  which  often  coats  the  grains  and  holds 

116  See  J.  A.  Phillipa  on  the  constitution  and  history  of  grits  and  sandstones. 
Quart.  Journ.  Geol.  Soc.  xxxvii.  (1881),  p.  6.  For  analyses  of  some  British 
sandstones  used  as  building  stones,  see  Wallace,  Proc.  Phil.  Soc.  Glasgow,  xiv. 
(1883),  p.  22. 


GEOGNOSY  231 

them  together  as  a  cement.  Iron,  the  great  coloring  ingre- 
dient of  rocks,  gives  rise  to  red,  brown,  yellow,  and  green 
hues,  according  to  its  degree  of  oxidation  and  hydration. 

Like  conglomerates,  sandstones  differ  in  the  nature  of 
their  component  grains,  and  in  that  of  the  cementing  matrix. 
Though  consisting  for  the  most  part  of  siliceous  grains,  they 
include  others  of  clay,  felspar,  mica,  zircon,  rutile,  tourma- 
line, or  other  minerals  such  as  occur  in  sand  (p.  227),  and 
these  may  increase  in  number  so  as  to  give  a  special  charac- 
ter to  the  rock.  Thus,  sandstones  may  be  argillaceous,  fel- 
spathic,  micaceous,  calcareous,  etc.  By  an  increase  in  the 
argillaceous  constituents,  a  sandstone  may  pass  into  one  of 
the  clay-rocks,  just  as  modern  sand  on  the  sea-floor  shades 
imperceptibly  into  mud.  On  the  other  hand,  by  an  augmen- 
tation in  the  size  and  sharpness  of  the  grains,  a  sandstone 
may  become  a  g  r  i  t,  and  by  an  increase  in  the  size  and  num- 
ber of  pebbles  may  pass  into  a  pebbly  or  conglomeratic  sand- 
stone, and  thence  into  a  fine  conglomerate.  A  piece  of  fine- 
grained sandstone,  seen  under  the  microscope,  looks  like  a 
coarse  conglomerate,  so  that  the  difference  between  the  two 
rocks  is  little  more  than  one  of  relative  size  of  particles. 

The  cementing  material  of  sandstones  may  be  ferrugi- 
nous, as  in  most  ordinary  red  and  yellow  sandstones,  where 
the  anhydrous  or  hydrous  iron-oxide  is  mixed  with  clay  or 
other  impurity — in  red  sandstones  the  grains  are  held  to- 
gether by  a  haematitic,  in  yellow  sandstones  by  a  limonitic 
cement;  argillaceous,  where  the  grains  are  united  by  a  base 
of  clay,  recognizable  by  the  earthy  smell  when  breathed 
upon;  calcareous,  where  carbonate  of  lime  occurs  either  as 
an  amorphous  paste  or  as  a  crystalline  cement  between  the 
grains;  siliceous,  where  the  component  particles  are  bound 
together  by  silica,  as  in  the  exposed  blocKs  of  Eocene  sand- 
stone known  as  "gray weathers"  in  Wiltshire,  and  which 
occur  also  over  the  north  of  France  toward  the  Ardennes. 

Among  the  varieties  of  sandstone  the  following  may  heje 
be  mentioned.  F 1  a  g  s  t  o  n  e — a  thin- bedded  sandstone, 
capable  of  being  split  along  the  lines  of  stratification  into 
thin  beds  or  flags;  Micaceous  sandstone  (mica-psam- 
mite) — a  rock  so  full  of  mica-flakes  that  it  splits  readily  into 
thin  laminae,  each  of  which  has  a  lustrous  surface  from  the 
quantity  of  silvery  mica.  This  rock  is  called  "fakes"  in 
Scotland.  Freeston e — a  sandstone  (the  term  being  ap- 
plied sometimes  also  to  limestone)  which  can  be  cut  into 
blocks  in  any  direction,  without  a  marked  tendency  to  split 
in  any  one  plane  more  than  in  another.  Though  this  rock 


232  TEXT-BOOK    OF    GEOLOGY 

occurs  in  beds,  each  bed  is  not  divided  into  laminae,  and  it 
is  the  absence  of  this  minor  stratification  which  makes  the 
stone  so  useful  for  architectural  purposes  (Craigleith  and 
other  sandstones  at  Edinburgh,  some  of  which  contain  98  per 
cent  of  silica).  Glauconitic  sandstone  (green-sand) 
— a  sandstone  containing  kernels  and  dusty  grains  of  glau- 
conite,  which  imparts  a  general  greenish  hue  to  the  rock. 
The  glauconite  has  probably  been  deposited  in  association 
with  decaying  organic  matter,  as  where  it  fills  echinus- 
spines,  foraminifera,  shells  and  corals  on  the  floor  of  the 
present  ocean.117  Buhrstone — a  highly  siliceous,  ex- 
ceedingly compact,  though  cellular  rock  (with  Chara  seeds, 
etc.),  found  alternating  with  unaltered  Tertiary  strata  in  the 
Paris  basin,  and  forming  from  its  hardness  and  roughness  an 
excellent  material  for  the  grindstones  of  flour-mills,  may  be 
mentioned  here,  though  it  probably  has  been  formed  by  the 
precipitation  of  silica  through  the  action  of  organisms.  Ar- 
k  o  s  e  (granitic  sandstone) — a  rock  composed  of  disintegrated 
granite,  and  found  in  geological  formations  of  different  ages, 
which  have  been  derived  from  granitic  rocks.  Crystal- 
lized sandstone — an  arenaceous  rock  in  which  a  de- 
posit of  crystalline  quartz  has  taken  place  upon  the  indi- 
vidual grains,  each  of  which  becomes  the  nucleus  of  a  more 
or  less  perfect  quartz  crystal.  Mr.  Sorby  has  observed  such 
crystallized  sand  in  deposits  of  various  ages  from  the  Oolites 
down  to  the  Old  Red  Sandstone.118 

Graywacke — a  compact  aggregate  of  rounded  or  subangular 
grains  of  quartz,  felspar,  slate,  or  other  minerals  or  rocks, 
cemented  by  a  paste  which  is  usually  siliceous,  but  may  be 
argillaceous,  felspathic,  calcareous,  or  anthracitic  (Fig.  21). 
Gray,  as  its  name  denotes,  is  the  prevailing  color:  but  it 
passes  into  brown,  brownish-purple,  and  sometimes,  where 
anthracite  predominates,  into  black.  The  rock  is  distin- 
guished from  ordinary  sandstone  by  its  darker  hue,  its  hard- 
ness, the  variety  of  it's  component  grains,  and,  above  all,  by 
the  compact  cement  in  which  the  grains  are  imbedded.  In 
many  varieties,  so  pervaded  is  the  rock  by  the  siliceous 
paste,  that  it  possesses  great  toughness,  and  its  grains  seem 
to  graduate  into  each  other  as  well  as  into  the  surrounding 


m  Ante,  p.  141 ;  Sollas,  Geol.  Mag.  iii.  2d  ser.  p.  539. 

118  Q.  J.  Geol.  Soc.  xxxvi.  p.  63.  See  Daubree,  Ann.  des  Mines,  2d  ser.  i. 
p.  206.  A.  A.  Young,  Amer.  Journ.  Sci.  3d  ser.  xxiii.  257 ;  xxiv.  47,  and  es- 
pecially the  work  of  Irving  and  Van  Hise  (quoted  on  p.  196),  which  gives  some 
excellent  figures  of  enlarged  quartz-graius. 


GEOGNOSY  233 

matrix.  Such  rocks,  when  fine-grained,  can  hardly,  at  first 
sight  or  with  the  unaided  eye,  be  distinguished  from  some 
compact  igneous  rocks,  though  a  microscopic  examination 
at  once  reveals  their  fragmental  character.  In  other  cases, 
where  the  graywacke  has  been  formed  mainly  out  of  the 
debris  of  granite,  quartz- porphyry,  or  other  felspathic 
masses,  the  grains  consist  so  largely  of  felspar,  and  the 

Easte  also  is  so  felspathic,  that  the  rock  might  be  mistaken 
}r  some  close-grained  granular  porphyry.  Graywacke  oc- 
curs extensively  among  the  Palaeozoic  formations,  in  beds 
alternating  with  shales  and  conglomerates.  It  represents 
the  muddy  sand  of  some  of  the  Palaeozoic  sea-floors,  retain- 
ing often  its  ripple-marks  and  sun-cracks.  The  metamor- 
phism  it  has  undergone  has  generally  not  been  great,  and 
lor  the  most  part  is  limited  to  induration,  partly  by  pres- 
sure and  partly  by  permeation  of  a  siliceous  cement.  But 
where  felspathic  ingredients  prevail,  the  rock  has  offered 
facilities  for  alteration,  and  has  been  here  and  there  changed 
into  highly  crystalline  mica-schists  full  of  garnets  and  other 
secondary  minerals  (contact-metamorphism  at  the  granite  of 
New  Galloway,  Scotland,  postea,  Book  IV.  Part  Viil.V 

The  more  fissile  fine-grained  varieties  of  this  rock  have 
been  termed  graywacke-slate  (p.  238).  In  these,  as  well  as 
in  graywacke,  organic  remains  occur  among  the  Silurian  and 
Devonian  formations.  Sometimes  in  the  Lower  Silurian 
rocks  of  Scotland,  these  strata  become  black  with  carbona- 
ceous matter,  among  which  vast  numbers  of  graptolites  may 
be  observed.  Gradations  into  sandstone  are  termed  Gray- 
wac  ke-s  a  n  d  s  ton  e.  In  Norway  the  reddish  felspathic 
graywacke  or  sandstone  of  the  Primordial  rocks  is  called 
Sparagmite;  similar  material  forms  much  of  the  Torri- 
don  sandstone  of  Scotland. 

Quartzite. — An  altered  siliceous  sandstone  (see  p.  311). 

2.  Clay  Rocks  (Pelites) 

These  are  composed  of  fine  argillaceous  sediment  or  mud, 
derived  from  the  waste  of  rocks.  Perfectly  pure  clay  or 
kaolin,  bydrated  silicate  of  alumina,  may  be  obtained  where 
granites  and  other  felspar-bearing  rocks  decompose.  But, 
as  a  rule,  the  argillaceous  materials  are  mixed  with  various 
impurities. 

Clay,  Mud. — The  decomposition  of  felspars  and  allied 
minerals  gives  rise  to  the  formation  of  hydrous  aluminous 
silicates,  which,  occurring  usually  in  a  state  of  fine  subdivi- 


234  TEXT-BOOK    OF   GEOLOGY 

siou,  are  capable  of  being  held  in  suspension  in  water,  and 
of  being  transported  to  great  distances.  These  substances, 
differing  much  in  composition,  are  embraced  under  the  gen- 
eral term  Clay,  which  may  be  defined  as  a  white,  gray, 
brown,  red,  or  bluish  substance,  which  when  dry  is  soft  arid 
friable,  adheres  to  the  tongue,  and  shaken  in  water  makes  it 
mechanically  turbid;  when  moist  is  plastic,  when  mixed 
with  much  water  becomes  mud.  It  is  evident  that  a  wide 
range  is  possible  for  varieties  of  this  substance.  The  fol- 
lowing are  the  more  important. 

Kaolin  (Porcelain-clay,  China-clay)  has  been  already  no- 
ticed (p.  140). 

Pipe-clay — white,  nearly  pure,  and  free  from  iron. 

Fire-clay — largely  found  in  connection  with  coal-seams, 
contains  little  iron,  and  is  nearly  free  from  lime  and  alka- 
lies. Some  of  the  most  typical  fire-clays  are  those  long  used 
as  Stourbridge,  Worcestershire,  for  the  manufacture  of  pot- 
tery. The  best  glass-house  pot-clay,  that  is,  the  most  re- 
fractory, and  therefore  used  for  the  construction  of  pots 
which  have  to  stand  the  intense  heat  of  a  glass-house,  has 
the  following  composition:  silica,  73 '82;  alumina,  15*88; 
protoxide  of  iron,  2 -95;  lime,  trace;  magnesia,  trace;  alka- 
lies, -90;  sulphuric  acid,  trace;  chlorine,  trace;  water,  6-45; 
specific  gravity,  2-51. 

Gannister — a  very  siliceous  close-grained  variety,  found  in 
the  Lower  Coal  measures  of  the  North  of  England,  and  now 
largely  ground  down  as  a  material  for  the  hearths  of  iron 
furnaces. 

Brick-clay — properly  rather  an  industrial  than  a  geological 
term,  since  it  is  applied  to  any  clay,  loam,  or  earth,  from 
which  bricks  or  coarse  pottery  are  made.  It  is  an  impure 
clay,  containing  a  good  deal  of  iron,  with  other  ingredients. 
An  analysis  gave  the  following  composition  of  a  brick-clay: 
silica,  49-44;  alumina,  34-26;  sesquioxide  of  iron,  7'74; 
lime,  1-48;  magnesia,  5 '14;  water,  1-94. 

Fuller's  Earth  (Terre  a  foulon,  Walkerde) — a  greenish  or 
brownish,  earthy,  soft,  somewhat  unctuous  substance,  with 
a  shining  streak,  which  does  not  become  plastic  with  water, 
but  crumbles  down  into  mud.  It  is  a  hydrous  aluminous 
silicate  with  some  magnesia,  iron-oxide  and  soda.  The  yel- 
low fuller's  earth  of  Reigate  contains  silica  44,  alumina  11, 
oxide  of  iron  10,  magnesia  2,  lime  5,  soda  5.'18  In  England 

119  tire's  Diet.  Arts,  etc.  ii.  p.  142. 


GEOGNOSY  235 

fuller's  earth  occurs  in  beds  among  the  Jurassic  and  Creta- 
ceous formations.  In  Saxony  it  is  found  as  a  result  of  the 
decomposition  of  diabase  and  gabbro. 

Wacke — a  dirty-green  to  brownish -black,  earthy  or  com- 
pact, but  tender  and  apparently  homogeneous  clay,  which 
arises  as  the  ultimate  stage  of  the  decomposition  of  basalt- 
rocks  in  situ. 

Loam — an  earthy  mixture  of  clay  and  sand  with  more  or 
less  organic  matter.  The  black  soils  of  Russia,  India,  etc. 
(Tchernosem,  Regur),  are  dark  deposits  of  loam  rich  in  or- 
ganic matter,  and  sometimes  upward  of  twenty  feet  deep. 

Loess — a  pale,  somewhat  calcareous  clay,  probably  of  wind- 
drift  origin,  found  in  some  river-valleys  (Rhine,  Danube, 
Mississippi,  etc.),  and  over  wide  regions  in  China  and  else- 
where. It  is  described  in  Book  III.  Part.  II.  Sect.  i.  §  1. 

Laterite — a  cellular,  reddish,  ferruginous  clay,  found  in 
some  tropical  countries  as  the  result  of  the  subaerial  decom- 
position of  rocks;  it  acquires  great  hardness  after  being 
quarried  out  and  dried. 

Till,  Bowlder-clay — a  stiff  sandy  and  stony  clay,  varying  in 
color  and  composition,  according  to  the  character  of  the 
rocks  of  the  district  in  which  it  lies.  It  is  full  of  worn 
stones  of  all  sizes,  up  to  blocks  weighing  several  tons,  and 
often  well-smoothed  and  striated.  It  is  a  glacial  deposit, 
and  will  be  described  among  the  formations  of  the  Glacial 
Period. 

Mudstone — a  fine,  usually  more  or  less  sandy,  argillaceous 
rock,  having  no  fissile  character,  and  of  somewhat  greater 
hardness  than  any  form  of  clay.  The  term  C  1  a-y-r  o  c  k  has 
been  applied  by  some  writers  to  an  indurated  clay  requiring 
to  be  ground  and  mixed  with  water  before  it  acquires  plas- 
ticity. 

Shale  (Schiste,  Schieferthon) — a  general  term  to  describe 
clay  that  has  assumed  a  thinly  stratified  or  fissile  structure. 
Under  this  term  are  included  laminated  and  somewhat  hard- 
ened argillaceous  rocks,  which  are  capable  of  being  split 
along  the  lines  of  deposit  into  thin  leaves.  They  present 
almost  endless  varieties  of  texture  and  composition,  passing, 
on  the  one  hand,  into  clays,  or,  where  much  indurated,  into 
slates  and  argillaceous  schists,  on  the  other,  into  flagstones 
and  sandstones,  or  again,  through  calcareous  gradations  into 
limestone,  or  through  ferruginous  varieties  into  clay-iron- 
stone, and  through  bituminous  kinds  into  coal. 

Clay-slate  (Schiste  ardoise,  ThonsehieferY — Under  this 
name  are  included  certain  hard  fissile  argillaceous  masses, 


236  TEXT-BOOK    OF   GEOLOGY 

composed  primarily  of  compact  clay,  sometimes  with  mega- 
scopic and  microscopic  scales  of  one  or  more  micaceous  min- 
erals, granules  of  quartz  and  cubes  or  concretions  of  pyrites, 
as  well  as  veins  of  quartz  and  calcite.  The  fissile  structure 
is  specially  characteristic.  In  some  cases  this  structure 
coincides  with  that,  of  original  deposit,  as  is  proved  by  the 
alteration  of  fissile  beds  with  bands  of  hardened  sandstone, 
conglomerate  or  fossiliferous  limestone.  But  for  the  most 
part  as  the  rocks  have  been  much  compressed,  the  fissile 
structure  of  the  argillaceous  bands  is  independent  of  stratifi- 
cation, and  can  be  seen  traversing  it.  Sorby  has  shown  that 
this  superinduced  fissility  or  " cleavage' '  has  resulted  from 
an  internal  rearrangement  of  the  particles  in  planes  perpen- 
dicular to  the  direction  in  which'  the  rocks  have  been  com- 
pressed (see  Book  III.  Part  I.  Section  iv.  §  3).  In  England 
the  term  "slate"  or  "clay-slate"  is  given  to  argillaceous,  not 
obviously  crystalline  rocks  possessing  this  cleavage -struc- 
ture, w  here  the  micaceous  lustre  of  the  finely  disseminated 
superinduced  mica  is  prominent,  the  rocks  are  phyllites. 

Microscopic  examination  shows  that  while  some  argilla- 
ceous rocks  consist  mainly  of  granular  debris,  many  cleaved 
clay-slates  contain  a  large  proportion  of  a  micaceous  mineral 
in  extremely  minute  flakes,  which  in  the  best  Welsh  slates 
have  an  average  size  of  2<Jx,  of  an  inch  in  breadth,  and  ^  of 
an  inch  in  thickness,  together  with  very  fine  black  hairs 
which  may  be  magnetite.1110  Moreover,  many  clay-slates, 
though  to  outward  appearance  thoroughly  noncrystalline, 
and  evidently  of  f ragmen tal  composition  and  sedimentary 
origin,  yet  contain,  sometimes  in  remarkable  abundance, 
microscopic  microlites  and  crystals  of  different  minerals 
placed  with  their  long  axes  parallel  with  the  planes  of  fis- 
sility. These  minute  bodies  include  yellowish- brown  nee- 
dles of  rutile,  greenish  or  yellowish  flakes  of  mica,  scales  of 
calcite,  and  probably  other  minerals.121  Small  granules  of 
quartz  containing  fluid-cavities,  show  on  their  surfaces  a 
distinct  blending  with  the  substance  of  the  surrounding 


120  Sorby,  Q.  J.  Geol.  Soc.  xxvi.  p.  68. 

121  These  "clay-slate  needles"  were  probably  not  crystallized  contemporane- 
ously with  the  deposit  of  the  original  rock.     In  some  cases  they  may  have  been 
deposited  with  the  rest  of  the  sediment  as  part  of  the  debris  of  pre-existing 
crystalline  rocks  (see  p.  228);  but  in  general  they  appear  to  have  been  devel- 
oped where  they  now  occur  by  subsequent  actions  (see  postea,  pp.  531,  632). 
For  their  character  see  Zirkel,  '"'Mik.  Beschaff."  p.  490.    Kalkowsky,  N.  Jahrb. 
1879,  p.  382;  A.  Cathrein,  op.  cit.   1882  (i.)  p.  169.     F.  Penck,   Sitzb.  Bayer. 
Akad.  Math.  Phys.  1880,  p.  461.     A.  Wichmann,  Q.  J.  Geol.  Soc.  xxxv.  p.  156. 


GEOGNOSY  237 

rock.  M.  Renard  has  found  that  the  Belgian  whet-slate  is 
full  of  minute  crystals  of  garnet.1"  Some  of  the  more  crys- 
talline varieties  (phyllite)  are  almost  wholly  composed  of 
minute  crystalline  particles  of  mica,  quartz,  felspar,  chlorite, 
and  rutile,  and  form  an  intermediate  stage  between  ordinary 
clay-slate  and  mica-schist. 

'A  distinction  has  been  drawn  by  some  petrographers  be- 
tween certain  rocks  (phyllite,  Urthonschiefer)  which  occur 
in  Archaean  regions  or  in  groups  probably  of  high  antiq- 
uity, and  others  (ardoise,  Thonschiefer)  which  are  found  in 
Palaeozoic  and  later  formations.  But  there  does  not  appear 
to  be  adequate  justification  for  this  grouping,  which  has 
probably  been  suggested  rather  by  theoretical  exigencies 
than  by  any  essential  differences  between  the  rocks  them- 
selves. That  the  whole  of  the  series  of  argillaceous  rocks, 
beginning  with  clay  and  passing  through  shale  into  slate 
and  pbyllite,  is  of  sedimentary  origin  is  indicated  by  the 
organic  remains,  false  bedding,  ripple-mark,  etc.,  found  in 
those  at  one  end  of  the  series,  and  by  the  insensible  grada- 
tion of  the  mineralogical  characters  through  increasing  stages 
of  metamorphism  to  the  other  end.  Some  microscopic  crys- 
tals may  possibly  have  been  originally  formed  among  the 
muddy  sediment  on  the  sea-floor  (see  p.  770).  Others  may 
have  formed  part  of  the  original  mechanical  detritus  that 
went  to  make  the  slate.  But,  for  the  most  part,  they  have 
been  subsequently  developed  within  the  rock,  and  represent 
early  stages  of  the  process  which  has  culminated  in  the  pro- 
duction of  crystalline  schists.  The  development  of  crystals 
of  chiastolite  and  other  minerals  in  clay-slate  is  frequently 
to  be  observed  round  bosses  of  granite,  as  one  of  the  phases 
of  contact-metamorphism. 

A  number  of  varieties  of  Clay-slate  are  recognized. 
Roofing  slate  (Dachschiefer)  includes  the  finest,  most 
compact,  homogeneous  and  durable  kinds,  suitable  for  roof- 
ing houses  or  the  manufacture  of  tables,  chimney-pieces, 
writing-slates,  etc. ;  it  occurs  in  the  Silurian  and  Devonian 
formations  of  Central  and  Western  Europe.  Anthra- 
ci  tic-si  ate  (anthracite-phyllite,  alum-slate),  dark  carbo- 
naceous slate  with,  much  iron-disulphide.  Bands  of  this 
nature  sometimes  run  through  a  clay-slate  region.  The 


128  Acad.  Roy.  Belgique,  xli.  (1877).  See  also  his  paper  on  the  composition 
and  structure  of  the  phyllades  of  the  Ardennes,  Bull.  Mus.  Roy.  Belg.  iii.  (1884), 
p.  281. 


238  TEXT- BOOK    OF   GEOLOGY 

carbonaceous  material  arises  from  the  alteration  of  the  re- 
mains of  plants  (fucoids)  or  animals  (frequently  graptolites). 
The  marcasite  so  abundantly  associated  with  these  organ- 
isms decomposes  on  exposure,  and  the  sulphuric  acid  pro- 
duced, uniting  with  the  alumina,  potash,  and  other  bases  of 
the  surrounding  rocks,  gives  rise  to  an  efflorescence  of  alum, 
or  the  decomposition  produces  sulphurous  springs  like  those 
of  Moffat.  The  name  Graywacke-slate  has  been  ap- 
plied to  extremely  fine-grained,  hard,  shaly,  more  or  less 
micaceous  and  sandy  bands,  associated  with  graywacke 
among  the  older  Palaeozoic  rocks.  Whet-slate,  Novae- 
ulite,  Hone-stone,  is  an  exceedingly  hard  fine-grained 
siliceous  rock,  some  varieties  of  which  derive  their  economic 
value  from  the  presence  of  microscopic  crystals  of  garnet. 
The  various  forms  of  altered  clay-slate  are  described  at  p.  309 
among  the  metamorphic  rocks. 

Porcellamte  (Ar^illite)  or  baked  shale — a  name  applied  to 
the  exceedingly  indurated  sometimes  partially  fused  condi- 
tion which  shales  are  apt  to  assume  in  contact  with  dikes 
and  intrusive  sheets  or  bosses.  For  an  account  of  this  form 
of  contact-metamorphism  see  Book  IV.  Part  VIII. 

3.   Volcanic  Fragmental  Rocks— Toffs 

This  section  comprises  all  deposits  which  have  resulted 
from  the  comminution  of  volcanic  rocks.  They  thus  in- 
clude (1)  those  which  consist  of  the  fragmentary  materials 
ejected  from  volcanic  foci,  or  the  true  ashes  and  tuffs ;  and 
(2)  some  rocks  derived  from  the  superficial  disintegration  of 
already  erupted  and  consolidated  volcanic  masses.  Obvi- 
ously the  second  series  ought  properly  to  be  classed  with 
the  sandy  or  clayey  rocks  above  described,  since  they  have 
been  formed  in  the  same  way.  In  practice,  however,  these 
detrital  reconstructed  rocks  cannot  always  be  certainly  dis- 
tinguished from  those  which  have  been  formed  by  the  con- 
solidation of  true  volcanic  dust  and  sand.  Their  chemical 
and  lithological  characters,  both  megascopic  and  micro- 
scopic, are  occasionally  so  similar,  that  their  respective 
modes  of  origin  have  to  be  decided  by  other  considera- 
tions, such  as  the  occurrence  of  lapilli,  bombs,  or  slags  in 
the  truly  volcanic  series,  and  of  well  water-worn  pebbles 
of  volcanic  rocks  in  the  other.  Attention  to  these  features, 
however,  usually  enables  the  geologist  to  make  the  distinc- 
tion, and  to  perceive  that  the  number  of  instances  where 
he  may  be  in  doubt  is  less  than  might  be  supposed.  Only 


GEOGNOSY  239 

a  comparatively  small  number  of  the  rocks  classed  here  are 
not  true  volcanic  ejections.123 

Referring  to  the  account  of  volcanic  action  in  Book  III. 
Part  I.  Sect,  i.,  we  may  here  merely  define  the  use  of  the 
names  by  which  the  different  kinds  of  ejected  volcanic 
materials  are  known. 

Volcanic  Blocks — angular,  sub-angular,  round,  or  irregu- 
larly-shaped masses  of  lava,  several  feet  in  diameter,  some- 
times of  uniform  texture  throughout,  as  if  they  were  large 
fragments  dislodged  by  explosion  from  a  previously  consoli- 
dated rock,  sometimes'  compact  in  the  interior  and  cellular 
or  slaggy  outside. 

Bombs — round,  elliptical,  or  discoidal  pieces  of  lava  from 
a  few  inches  up  to  one  or  more  feet  in  diameter.  They  are 
frequently  cellular  internally,  while  the  outer  parts  are  fine- 
grained. Occasionally  they  consist  of  a  mere  shell  of  lava 
with  a  hollow  interior  like  a  bombshell,  or  of  a  casing  of 
lava  inclosing  a  fragment  of  rock.  Their  mode  of  origin 
is  explained  in  Book  III.  Part  I.  Sect.  i.  §  1. 

Lapilli  (rapilli) — ejected  fragments  of  lava,  round,  angular, 
or  indefinite  in  shape,  varying  in  size  from  a  pea  to  a  walnut. 
Their  mineralogical  composition  depends  upon  that  of  the 
lava  from  which  they  have  been  thrown  up.  Usually  they 
are  porous  or  finely  vesicular  in  texture. 

Volcanic  Sand,  Volcanic  Ash — the  finer  detritu's  erupted  from 
volcanic  orifices,  consisting  partly  of  rounded  and  angular 
fragments  up  to  about  the  size  of  a  pea  derived  from  the 
explosion  or  lava  within  eruptive  vents,  partly  of  vast 
quantities  of  microlites  and  crystals  of  some  of  the  min- 
erals of  the  lava.  The  finest  dust  is  in  a  state  of  extremely 
minute  subdivision.  When  examined  under  the  microscope, 
it  is  sometimes  found  to  consist  not  only  of  minute  crystals 
and  microlites,  but  of  volcanic  glass,  which  may  be  observed 
adhering  to  the  microlites  or  crystals  round  which  it  flowed 
when  still  part  of  the  fluid  lava.  The  presence  of  minutely 
cellular  fragments  is  characteristic  of  most  volcanic  f rag- 
mental  rocks,  and  this  structure  may  commonly  be  observed 
in  the  microscopic  fragments  and  filaments  of  glass. 

When  these  various  materials  are  allowed  to  accumulate, 
they  become  consolidated  and  receive  distinctive  names. 
In  cases  where  they  fall  into  the  sea  or  into  lakes,  they  are 
liable  at  the  outer  margin  of  their  area  to  be  mingled  with, 

143  For  a  classification  of  tuffs  and  tuffaceous  deposits  see  E.  Reyer,  Jakrb. 
K.  K.  Geol.  Reichsanst.  xxxi.  (1881),  p.  51. 


2-iO  TEXT-BOOK    OF   GEOLOGY 

and  insensibly  to  pass  into  ordinary  non-volcanic  sediment. 
Hence  we  may  expect  to  find  transitional  varieties  between 
rocks  formed  directly  from  the  results  of  volcanic  explosion 
and  ordinary  sedimentary  deposits. 

Volcanic  Conglomerate — a  rock  composed  mainly  or  entirely 
of  rounded  or  sub-angular  fragments,  chiefly  or  wholly  of 
volcanic  rocks,  in  a  paste  of  the  same  materials,  usually 
exhibiting  a  stratified  arrangement,  and  often  found  inter- 
calated between  successive  sheets  of  lava.  Conglomerates 
of  this  kind  may  have  been  formed  by  the  accumulation  of 
rounded  materials  ejected  from  volcanic  vents;  or  as  the 
result  of  the  aqueous  erosion  of  previously  solidified  lavas, 
or  by  a  combination  of  both  these  processes.  Well-rounded 
and  smoothed  stones  almost  certainty  indicate  long-continued 
water-action,  rather  than  trituration  in  a  volcanic  vent.  In 
the  Western  Territories  of  the  United  States  vast  tracts  of 
country  are  covered  with  masses  of  such  conglomerate, 
sometimes  2000  feet  thick.  Captain  Button  has  shown  that 
similar  deposits  are  in  course  of  formation  there  now,  merely 
by  the  influence  of  disintegration  upon  exposed  lavas.'34 

Volcanic  conglomerates  receive  different  names  accord- 
ing to  the  nature  of  the  component  fragments;  thus  we  have 
basalt-conglomerates,  where  these  fragments  are  wholly  or 
mainly  of  basalt,  trachyte -conglomerates,  porphy  rite -conglom- 
erates, phonolite- conglomerates,  etc. 

Volcanic  Breccia  resembles  Volcanic  Conglomerate,  except 
that  the  stones  are  angular.  This  angularity  indicates  an 
absence  of  aqueous  erosion,  and,  under  the  circumstances 
in  which  it  is  found,  usually  points  to  immediately  adjacent 
volcanic  explosions.  There  is  a  great  variety  of  breccias, 
as  basalt-breccia,  diabase-breccia,  etc. 

Volcanic  Agglomerate — a  tumultuous  assemblage  of  blocks  of 
all  sizes  up  to  masses  several  yards  in  diameter,  met  with 
in  the  "necks"  or  pipes  of  old  volcanic  orifices.  The  stones 
and  paste  are  commonly  of  one  or  more  volcanic  rocks, 
such  as  felsite,  porphyrite  or  basalt,  but  they  include  also 
fragments  of  the  surrounding  rocks,  whatever  these  may 
be,  through  which  the  volcanic  orifice  has  been  drilled. 
As  a  rule,  agglomerate  is  devoid  of  stratification;  but  some- 
times it  includes  portions  which  have  a  more  or  less  distinct 
arrangement  into  beds  of  coarser  and  finer  detritus,  often 
placed  on  end,  or  inclined  in  different  directions  at  high 
angles,  as  described  in  Book  IV.  Part  VII.  Sect.  i.  §  4. 

184  "High  Plateaus  of  Utah,"  p.  77. 


GEOGNOSY  241 

Volcanic  Tuff. — This  general  term  may  be  made  to  include 
all  the  finer  kinds  of  volcanic  detritus,  ranging,  on  the  one 
hand,  through  coarse  gravelly  deposits  into  conglomerates, 
and  on  the  other,  into  exceedingly  compact  fine-grained 
rocks,  formed  of  the  finest  and  most  impalpable  kind  of 
volcanic  dust.  Some  modern  tuffs  are  full  of  microlites, 
derived  from  the  lava  which  was  blown  into  dust.  Others 
are  formed  of  small  rounded  or 
angular  grains  of  different  lavas, 
with  fragments  of  various  rocks 
through  which  the  volcanic  fun- 
nels have  been  drilled.  The  tuffs 
of  earlier  geological  periods  have 
often  been  so  much  altered,  that 
it  is  difficult  to  state  what  may 
have  been  their  original  condi- 
tion. The  absence  of  microlites 
and  glass  in  them  is  no  proof 
that  they  are  not  true  tuffs;  for 
the  presence  of  these  bodies  de-  plg  w._Mtewsoplo  structure  * 

pends     Upon     the      nature     of     the  Carboniferous   Palagonite  Tuff 

lavas.      If    the    latter    were    not 

vitreous  and  microlitic,  neither  would  be  the  tuffs  derived 
from  them.  In  the  Carboniferous  volcanic  area  of  Central 
Scotland,  the  tuffs  are  made  up  of  debris  and  blocks  of 
the  basaltic  lavas,  and,  like  these,  are  not  microlitic,  though 
in  some  places  they  abound  in  fragments  of  the  basic  glass 
called  palagonite.  (Fig.  23,  and  infra,  p.  242.) 

Tuffs  have  consolidated  sometimes  under  water,  some- 
times on  dry  land.  As  a  rule,  they  are  distinctly  stratified. 
Near  the  original  vents  of  eruption  they  commonly  present 
rapid  alternations  of  finer  and  coarser  detritus,  indicative  of 
successive  phases  of  volcanic  activity.  They  necessarily 
shade  off  into  the  sedimentary  formations  with  which  they 
were  contemporaneous.  Thus,  we  have  tuffs  passing  grad- 
ually into  shale,  limestone,  sandstone,  etc.  The  interme- 
diate varieties  have  been  called  ashy  shale,  tuffaceous  shale, 
or  shaly  tuff,  etc.  From  the  circumstances  of  their  forma- 
tion, tuffs  frequently  preserve  the  remains  of  plants  and 
animals,  both  terrestrial  and  aquatic.  Those  of  Monte 
Somma  contain  fragments  of  land-plants  and  shells.  Some 
of  those  of  Carboniferous  age  in  Central  Scotland  have 

E "elded  crinoids,  brachiopods,  and  other  marine  organisms. 
ike  the  other  fragmentary  volcanic  rocks,  the  tuffs  may 
be  subdivided  according  to  the  nature  of  the  lava  from  the 
GEOLOGY— Vol.  XXIX— 11 


F, 


242  TEXT-BOOK   OF   GEOLOGY 

disintegration  of  which  they  have  been  formed.  Thus  we 
have  felsite-tuffs,  trachyte-tuffs,  basalt-tuffs,  pumice-tuffs,  por- 
phyrite-tuffs,  etc.  A  few  varieties  with  special  characteris- 
tics may  be  mentioned  here.1" 

Trass — a  pale  yellow  or  gray  rock,  rough  to  the  feel,  com- 
osed  of  an  earthy  or  compact  pumiceous  dust,  in  which 
fragments  of  pumice,  trachyte,  graywacke,  basalt,  carbon- 
ized wood,  etc.,  are  imbedded.  It  has  filled  up  some  of  the 
valleys  of  the  Eifel,  where  it  is  largely  quarried  as  a  hy- 
draulic mortar. 

Peperino — a  dark-brown,  earthy  or  granular  tuff,  found  in 
considerable  quantity  among  the  Alban  Hills  near  Borne, 
and  containing  abundant  crystals  of  augite,  mica,  leucite, 
magnetite,  and  fragments  of  crystalline  limestone,  basalt, 
and  leucite-lava. 

Palagonite-Tuff — a  bedded  aggregate  of  dust  and  fragments 
of  basaltic  lava,  among  which  are  conspicuous  angular 
pieces  and  minute  granules  of  the  pale  yellow,  green,  red, 
or  brown  basic  glass  called  palagonite.  This  vitreous  sub- 
stance is  intimately  related  to  the  basalts  (p.  298).  It  ap- 
pears to  have  gathered  within  volcanic  vents  and  to  have 
been  emptied  thence,  not  in  streams,  but  by  successive 
aeriform  explosions,  and  to  have  been  subsequently  more 
or  less  altered.  The  percentage  composition  of  a  specimen 
from -the  typical  locality,  Palagonia,  in  the  Val  d.i  Noto, 
Sicily,  was  estimated  by  Sartorius  von  Waltershausen  to 
be:  silica,  41-26;  alumina,  8-60;  ferric  oxide,  25-32;  lime, 
5-59;  magnesia,  4-84;  potash,  0-54;  soda,  1-06;  water,  12-79. 
This  rock  is  largely  developed  among  the  products  of  the 
Icelandic  and  Sicilian  volcanoes;  it  occurs  also  in  the  Eifel 
and  in  Nassau.  It  has  been  found  to  be  one  of  the  charac- 
teristic features  of  tuffs  of  Carboniferous  age  in  Central 
Scotland188  (Fig.  23). 

Schalstem. — Under  this  name,  German  petrographers  have 
placed  a  variety  of  green,  gray,  red,  or  mottled  fissile  rocks, 
impregnated  with  carbonate  of  lime.  They  are  inter-strati- 
fied with  the  Devonian  formations  of  Nassau,  the  Harz  and 

196  On  the  occurrence  and  structure  of  tuffs,  see  J.  C:  Ward,  Q.  J.  Geol.  Soc. 
xxxi.  p.  388;  Reyer,  Jahrb.  Geol.  Reichsanst.  1881,  p.  57;  Geikie,  Trans.  Roy. 
Soc.  Edin.  xxix. ;  Vogelsang,  Z.  Deutsch.  Geol.  Ges.  xxiv.  p.  643 ;  Penck,  op. 
cit.  xxxi.  p.  504.  On  the  basalt-tuffs  of  Scania,  F.  Eichstadt,  Sveriges  Geol. 
Undersokn,  ser.  c.  No.  58  (1883).  On  the  metamorphism  of  tuffs  into  lava-like 
rocks,  see  Button's  "High  Plateaux  of  Utah"  (U.  S.  Geograph.  and  Geol.  Sur- 
vey of  Rocky  Mounts.),  1880,  p.  79. 

156  Trans.  Roy.  Soc.  Edin.  xxix.  p.  514. 


GEOGNOSY  243 

Devonshire,  and  with  the  Silurian  rocks  of  Bohemia.  They 
sometimes  contain  fragments  of  clay-slate,  and  are  occasion- 
ally fossiliferous.  They  present  amygdaloidal  and  porphy- 
ritic,  as  well  as  perfectly  laminated  structures.  Probably 
they  are  in  most  cases  true  diabase-tuffs,  but  sometimes 
they  may  be  forms  of  diabase-lavas,  which,  like  the  strati- 
fied formations  in  which  they  lie,  have  undergone  altera- 
tion, and  in  particular  have  acquired  a  more  or  less  dis- 
tinctly fissile  structure,  as  the  result  of  lateral  pressure  and 
internal  crushing. 1ST 

4.   Fragmental  Rocks  of  Organic  Origin 

This  series  includes  deposits  formed  either  by  the  growth 
and  decay  of  organisms  in  situ,  or  by  the  transport  and  sub- 
sequent accumulation  of  their  remains.  These  may  be  con- 
veniently grouped,  according  to  their  predominant  chemical 
ingredient,  into  Calcareous,  Siliceous,  Phosphatic,  Carbona- 
ceous, and  Ferruginous. 

1.  CALCAREOUS. — Besides  the  calcareous  formations 
which  occur  among  the  stratified  crystalline  rocks  as  re- 
sults of  the  deposition  of  chemical  precipitates  (p.  260), 
a  more  important  series  is  derived  from  the  remains  of 
living  organisms,  either  by  growth  on  the  spot  or  by  trans- 
port and  accumulation  as  mechanical  sediment.  To  by  far 
the  larger  part  of  the  limestones  intercalated  in  the  rocky 
framework  of  our  continents,  an  organic  origin  may  with 
probability  be  assigned.  It  is  true,  as  has  been  above  men- 
tioned (p.  216),  that  limestone,  formed  of  the  remains  of 
animals  or  plants,  is  liable  to  an  internal  crystalline  re- 
arrangement, the  effect  of  which  is  to  obliterate  the  organic 
structure.  Hence  in  many  of  the  older  limestones  no  trace 
of  any  fossils  can  be  detected,  and  yet  these  rocks  were 
almost  certainly  formed  of  organic  remains.  An  attentive 
microscopic  study  of  organic  calcareous  structures,  and ,  of 
the  mode  of  their  replacement  by  crystalline  calcite,  some- 
times detects  indications  of  former  organisms,  even  in  the 
midst  of  thoroughly  crystalline  materials."8 


IOT  0.  Koch,  Jahrb.  Ver.  Nat.  Nassau,  xiii.  (1858),  216,  238.  J.  A.  Phillips, 
Q.  J.  Geol.  Soc.  xxxii.  p.  155,  xxxiv.  p.  471. 

128  Sorby,  Address  to  Geol.  Society,  February,  1879,  and  the  paper  of  Messrs. 
Cornish  and  Kendall,  cited  ante,  p.  216.  Gumbel  has  suggested  that  the  different 
durability  of  the  calcite  and  aragonite  organic  forms  may  be  due  rather  to  struc- 
ture than  mineral  composition. 


244  TEXT-BOOK   OF   GEOLOGY 

Limestone,  composed  of  the  remains  of  calcareous  organ- 
isms, is  found  in  layers  which  range  from  mere  thin  larninaa 
up  to  massive  beds  several  feet  or  even  yards  in  thickness. 
In  some  instances,  such  as  that  of  the  Carboniferous  or 
Mountain  limestone  of  Britain  and  Belgium,  and  that  of  the 
Coal-measures  in  Wyoming  and  Utah,  it  occurs  in  continu- 
ous superposed  beds  to  a  united  thickness  of  several  thou- 
sand feet,  and  extends  for  hundreds  of  square  miles,  forming 
a  rock  out  of  which  picturesque  gorges,  hills,  and  table-lands 
have  been  excavated. 

Limestones  of  organic  origin  present  every  gradation  of 
texture  and  structure,  from  mere  soft  calcareous  mud  or 
earth,  evidently  composed  of  entire  or  crumbled  organisms, 
up  to  solid  compact  crystalline  rock,  in  which  indications 
of  an  organic  source  can  hardly  be  perceived.  Mr.  Sorby, 
in  the  address  already  cited,  called  renewed  attention  to  the 
importance  of  the  form  in  which  carbonate  of  lime  is  built 
up  into  animal  structures.  Quoting  the  opinion  of  Rose  ex- 
pressed in  1858,  that  the  diversity  in  the  state  of  preserva- 
tion of  different  shells  might  be  due  to  the  fact  that  some  of 
them  had  their  lime  as  calcite,  others  as  aragonite,  he  showed 
that  this  opinion  is  amply  supported  by  microscopic  exami- 
nation. Even  in  the  shells  of  a  recent  raised  beach,  he  ob- 
served that  the  inner  aragonite  layer  of  the  common  mussel 
had  been  completely  removed,  though  the  outer  layer  of  cal- 
cite was  well  preserved.  In  some  shelly  limestones  contain- 
ing casts,  the  aragonite  shells  have  alone  disappeared,  and 
where  these  still  remain  represented  by  a  calcareous  layer, 
this  has  no  longer  the  original  structure,  but  is  more  or  less 
coarsely  crystalline,  being  in  fact  a  pseudomorph  of  calcite 
after  aragonite,  and  quite  unlike  contiguous  calcite  shells, 
which  retain  their  original  microscopical  and  optical  char- 
acters.18* 

The  following  list  comprises  some  of  the  more  distinctive 
and  important  forms  of  organically-derived  limestones. 

S  h  e  1 1-M  a  r  1 — a  soft,  white,  earthy,  or  crumbling  de- 
posit, formed  in  lakes  and  ponds  by  the  accumulation  of  the 
remains.of  shells  and  Entomostraca  on  the  bottom.  When 
such  calcareous  deposits  become  solid  compact  stone  they 
are  known  as  fresh-water  (lacustrine)  limestones.  These  are 
generally  of  a  smooth  texture,  and  either  dull  white,  pale 

129  The  student  will  find  the  address  from  which  these  citations  are  made 
lull  of  suggestive  matter  in  regard  to  the  origin  and  subsequent  history  of  lime- 
stones. 


GEOGNOSY 


245 


gray,  or  cream-colored,  their  fracture  slightly  conchoidal, 
rarely  splintery. 

Lumachell e — a  compact,  dark  gray  or  brown  lime- 
stone, charged  with  ammonites  or  other  fossil  shells,  which 
are  sometimes  iridescent,  giving  bright  green,  blue,  orange, 
and  dark  red  tints  (fire-marble). 

Calcareous  (Foraminiferal)  Ooz e — a  white  or 
gray  calcareous  mud,  of  organic  origin,  found  covering  vast 
areas  of  the  floor  of  the  Atlantic  and  other  oceans,  and 
formed  mostly  of  the  remains  of  Foraminif&ra,  particularly 


Pig.  34.— Foraminiferal  (Globigerina)  Ooze,  dredged  by  the  "Challenger" 

Expedition  in  Lat.  50°  1'  S.,  Long.  123°  V  E.,  from  a  depth  of  1800 

fathoms  (magnified  50  Diameters). 

of  forms  of  the  genus  Globigerina  (Fig.  24).  Further  ac- 
count of  this  and  other  organic  deep-sea  deposits  is  given  in 
Book  III.  Part  II.  Section  iii. 

S  h  e  1 1-S  a  n  d — a  deposit  composed  in  great  measure  or 
wholly  of  comminuted  shells,  found  commonly  on  a  low 
shelving  coast  exposed  to  prevalent  on-shore  winds.  When 
thrown  above  the  reach  of  the  waves  and  often  wetted  by 
rain,  or  by  trickling  runnels  of  water,  it  is  apt  to  become 
consolidated  into  a  mass,  owing  to  the  solution  and  rede- 
posit  of  lime  round  the  grains  of  shell  (p.  216). 

Coral-rock — a   limestone   formed   by  the  continuous 


246  TEXT-BOOK    OF   GEOLOGY 

growth  of  coral-building  polyps.  This  substance  affords 
an  excellent  illustration  01  the  way  in  which  organic  struc- 
ture may  be  effaced  from  a  limestone  entirely  formed  of  the 
remains  of  once  living  animals.  Though  the  skeletons  of 
the  reef -building  corals  remain  distinct  on  the  upper  sur- 
face, those  of  their  predecessors  beneath  them  are  gradually 
obliterated  by  the  passage  through  them  of  percolating  water, 
dissolving  and  redepositing  calcium  carbonate.  We  can 
thus  understand  how  a  mass  of  crystalline  limestone  may 
have  been  produced  from  one  formed  out  of  organic  re- 
mains, without  the  action  of  any  subterranean  heat,  but 
merely  by  the  permeation  of  water  from  the  surface.180 

C  halk — a  white  soft  rock,  meagre  to  the  touch,  soiling 
the  fingers,  formed  of  a  fine  calcareous  flour  derived  from 
the  remains  of  Foraminifera,  echinoderms,  mollusks,  and 
other  marine  organisms.  By  making  thin  slices  of  the  rock 
and  examining  them  under  the  microscope,  Sorby  has  found 
that  Foraminifera,  particularly  Globigerina,  and  single  de- 
tached cells  of  comparatively  shallow-water  forms,  probably 
constitute  less  than  half  of  the  rock  by  bulk  (Fig.  22),  the 
remainder  consisting  of  detached  prisms  of  the  outer  calcare- 
ous layer  of  Inoceramus,  fragments  of  Ostrea,  Pecten,  echino- 
derms, spicules  of  sponges,  etc.  It  is  not  quite  like  any  known 
modern  deep-sea  deposit.  A  microscopic  investigation  of 
chalk  from  the  neighborhood  of  Lille  showed  that,  besides 
the  usual  organic  constituents,  the  rock  contains  minute 
grains  and  crystals  of  quartz,  tourmaline,  zircon,  rutile, 
garnet  and  felspars,131  these  minerals  being  among  the  most 
widely  diffused  and  persistent  ingredients  in  the  finer  sedi- 
ments that  are  derived  from  the  denudation  of  crystalline 
rocks  (see  p.  228). 

Crinoidal  (Encrinite)  Limeston  e — a  rock  com- 
posed in  great  part  of  crystalline  joints  of  encrinites,  with 
Foraminifera,  corals,  and  mollusks.  It  varies  in  color  from 
white  or  pale  gray,  through  shades  of  bluish-gray  (sometimes 
yellow  or  brown,  less  commonly  red)  to  a  dark  gray  or  even 
black  color.  It  is  abundant  among  Palaeozoic  formations, 
being  in  Western  Europe  especially  characteristic  of  the 
lower  part  of  the  Carboniferous  system. 


130  See  Dana's  "Coral  and  Coral  Islands,"  p.  354;  also  the  account  of  the 
Devonian  and  Carboniferous  limestones  in  the  present  volume.  Dupont  has 
shown  that  many  of  the  massive  limestones  of  Belgium  have  been  formed  by 
reef -like  masses  of  Stromatopora  or  allied  organisms. 

181  L.  Cayeux,  Ann.  Soc.  Geol.  Nord.  xvii.  (1890),  p.  283. 


GEOGNOSY  247 

2.  SILICEOUS. — Silica  is  directly  eliminated  from  both 
fresh  and  salt  water  by  the  vital  growth  of  plants  and  ani- 
mals. (Book  TIL  Part  II.  Section  iii.) 

Diatom-earth,  Tn polite  (Infusorial  earth,  Kieselguhr) — a  sili- 
ceous deposit  formed  chiefly  of  the  frustules  of  diatoms,  laid 
down  both  in  salt  and  in  fresh  water.  Wide  areas  of  it  are 
now  being  deposited  on  the  bed  of  the  South  Pacific  (Dia- 
tom-ooze, Fig.  181).  In  Virginia,  United  States,  an  exten- 
sive tract  occurs  covered  with  diatom-earth  to  a  depth  of  40 
feet.  It  likewise  underlies  peat-mosses,  probably  as  an 
original  lake-deposit.  It  is  used  as  Tripoli  powder  for  pol- 
ishing purposes  (see  p.  807). 

Radiolanan  ooze — a  pale  chalk-like  abysmal  marine  deposit 
consisting  mainly  of  the  remains  of  siliceous  radiolarians  and 
diatoms.  It  is  further  referred  to  in  Book  III.  Part  II. 
Section  iii. 

Flint  (Silex,  Feuerstein) — a  .gray  or  black,  excessively 
compact  rock,  with  the  hardness  of  quartz  and  a  perfect 
conchoidal  fracture,  its  splinters  being  translucent  on  the 
edges.  Consists  of  an  intimate  mixture  of  crystalline  in- 
soluble silica  and  of  amorphous  silica  soluble  in  caustic 
potass.  Its  dark  color,  which  can  be  destroyed  by  heat, 
arises  chiefly  from  the  presence  of  carbonaceous  matter. 
Flint  occurs  principally  as  nodules,  dispersed  in  layers 
through  the  Upper  Chalk  of  England  and  the  northwest  of 
Europe.  It  frequently  incloses  organisms  such  as  sponges, 
echini  and  brachiopods.  It  has  been  deposited  from  sea- 
water,  at  first  through  organic  agency,  and  subsequently  by 
direct  chemical  precipitation  round'  the  already  deposited 
silica.  (Book  III.  Part  II.  Section  iii.)  Chert  (phtanite) 
is  a  name  applied  to  impure  calcareous  varieties  of  flint,  in 
layers  and  nodules  which  are  found  among  the  Palaeozoic 
and  later  formations,  especially  but  not  exclusively  in  lime- 
stones.132 In  some  cases,  as  in  the  spicules  of  sponges,  the 
silica  has  had  a  directly  organic  origin,  having  been  secreted 
from  sea- water  by  the  living  organisms;  in  other  cases, 
where,  for  example,  we  find  a  calcareous  shell,  or  echinus, 
or  coral  converted  into  silica,  it  would  seem  that  the  substi- 
tution of  silica  for  calcium-carbonate  has  been  effected  by  a 


139  Consult  Hull  and  Hardman,  Trans.  Roy.  Dublin  Soc.  i.  (1878),  p.  71. 
Renard,  Bull.  Acad.  Roy.  Belgique,  2d  ser.  vol.  xlvi.  p.  471;  Sollas,  Ann. 
Mag.  Nat.  Hist.  vii.  (1881),  p.  141 ;  Scientific  Proc.  Roy.  Dublin  Soc.  vi.  (1887), 
part.  i.  G.  J.  Hinde,  Geol.  Mag.  1887,  p.  43fi.  Bands  of  radiolarian  chert  oc- 
cupy persistent  horizons  among  the  Lower  Silurian  rocks  of  southern  Scotland. 


248  TEXT-BOOK    OF   GEOLOGY 

process  of  chemical  pseudomorphism,  either  after  or  during 
the  formation  of  the  limestone.  The  vertical  ramifying 
masses  of  flint  in  Chalk  show  that  the  calcareous  ooze  had 
to  some  extent  accumulated  before  the  segregation  of  these 
masses.188 

3.  PHOSPHATIC. — A  few  invertebrata  contain  phosphate 
of  lime.  Among  these  may  be  mentioned  the  brachiopoda 
Lingula  and  Orbicula,19*  also  Conularia,  Serpulites,  and  some 
recent  and  fossil  Crustacea.  The  shell  of  the  recent  Lingula 
ovalis  was  found  by  Hunt  to  contain,  after  calcination,  61 
per  cent  of  fixed  residue,  which  consisted  of  85-70  per  cent 
of  phosphate  of  lime;  11-75  carbonate  of  lime,  and  2-80  mag- 
nesia. The  bones  of  vertebrate  animals  likewise  contain 
about  60  per  cent  of  phosphate  of  lime,  while  their  excre- 
ment sometimes  abounds  in  the  same  substance.  Hence  de- 
posits rich  in  phosphate  of  lime  have  resulted  from  the  ac- 
cumulation of  animal  remains  from  Silurian  times  up  to  the 
present  day.  Associated  with  the  Bala  limestone,  in  the 
Lower  Silurian  series  of  North  Wales,  is  a  band  composed 
of  concretions  cemented  in  a  black,  graphitic,  slightly  phos- 
phatic  matrix,  and  containing  usually  64  per  cent  of  phos- 
phate of  lime  (phosphorite).136  The  tests  of  the  trilobites  and 
other  organisms  among  the  Cambrian  rocks  of  Wales  also 
contain  phosphate  of  lime,  sometimes  to  the  extent  of  20  per 
cent.138  Phosphatic,  though  certainly  far  inferior  in  extent 
and  importance  to  calcareous,  and  even  to  siliceous,  forma- 
tions, are  often"  of  singular  geological  interest.  The  follow- 
ing examples  may  serve  as  illustrations.137 

Cuano — a  deposit  consisting  mainly  of  the  droppings  of 
sea-fowl,  formed  on  islands  in  rainless  tracts  off  the  western 
coasts  of  South  America  and  of  Africa.  It  is  a  brown,  light, 
powdery  substance  with  a  peculiar  ammoniacal  odor,  and 
occurs  in  deposits  sometimes  more  than  100  feet  thick. 
Analyses  of  American  guano  give — combustible  organic 
matter  and  acids,  11-3;  ammonia  (carbonate,  urate,  etc.), 
31-7;  fixed  alkaline  salts,  sulphates,  phosphates,  chlorides, 
etc.,  8-1;  phosphates  of  lime  and  magnesia,  22-5;  oxalate  of 
lime,  2-6;  sand  and  earthy  matter,  1-6;  water,  22-2.  This 

133  On  formation  of  chalk- flints,  see  Book  III.  Part  II.  Section  iii.  §  3. 

134  Sterry  Hunt,  Amer.  Journ.  Soc.  xvii.  (1854),  p.  236.     Logan's  ''Geology 
of  Canada,"  1863,  p.  461. 

136  D.  C.  Davies,  Q.  J.  Geol.  Soc.  xxxi.  p.  357.     13«  Hicks,  op.  cit.  p.  368. 

131  For  an  exhaustive  account  of  deposits  of  phosphate  of  lime,  see  R.  A.  P. 
Penrose,  Jr. ;  Bull.  U.  S.  Geol.  Surv.  No.  46,  1888,  also  postea,  Book  III.  Part 
II.  Sect.  iii.  §  3. 


GEOGNOSY  249 

remarkable  substance  is  highly  valuable  as  a  source  of  arti- 
ficial manures.  (Book  III.  Part  II.  Section  iii.) 

Bone-Breccia — a  deposit  consisting  largely  of  fragmentary 
bones  of  living  or  extinct  species  of  mammalia,  found  some- 
times under  stalagmite  on  the  floors  of  limestone  caverns, 
more  or  less  mixed  with  earth,  sand,  or  lime.  In  some  older 
geological  formations,  bone-beds  occur,  formed  largely  of 
the  remains  of  reptiles  or  fishes,  as  the  "Lias  bone-bed," 
and  the  "Ludlow  bone-bed." 

Coprolitic  nodules  and  beds138 — are  formed  of  the  accumu- 
lated excrement  (coprolites)  of  vertebrated  animals.  Among 
the  Carboniferous  shales  01  the  basin  of  the  Firth  of  Forth, 
coprolitic  nodules  are  abundant,  together  with  the  bones  and 
scales  of  the  larger  ganoid  fishes  which  voided  them:  abun- 
dance of  broken  scales  and  bones  of  the  smaller  ganoids  can 
usually  be  observed  in  the  coprolites.  Among  the  Lower 
Silurian  rocks  of  Canada,  numerous  phosphatic  nodules, 
supposed  to  be  of  coprolitic  origin,  occur.13*  The  phos- 
phatic beds  of  the  Cambridgeshire  Cretaceous  rocks  are  now 
largely  worked  as  a  source  of  artificial  manure.  In  popular 
and  especially  commercial  usage,  the  word  "coprolitic"  is 
applied  to  nodular  deposits  which  can  be  worked  for  phos- 
phate of  lime,  thougn  they  may  contain  few  or  no  true 
coprolites. 

Phosphatic  Chalk.— In  the  Chalk  of  France  and  Belgium, 
more  sparingly  in  that  of  England,  certain  lavers  occur 
where  the  original  calcareous  matter  has  been  replaced  to 
a  considerable  extent  by  phosphate  of  lime.  Such  bands 
have  frequently  a  brownisn  tint,  which  on  examination  is 
found  to  result  from  the  abundance  of  minute  brown  grains 
composed  mainly  of  phosphate.  The  foraminifera  and  other 
minuter  or  fragmentary  fossils  have  been  changed  into  this 
brown  substance.  The  proportion  of  phosphate  of  lime 
ranges  up  to  45  per  cent  or  more.140 

4.  CARBONACEOUS. — The  formations  here  included^have 
almost  always  resulted  from  the  decay  and  entombment  of 
vegetation  on  the  spot  where  it  grew,  sometimes  by  the 
drifting  of  the  plants  to  a  distance  and  their  consolidation 


188  On  the  origin  of  phosphatic  nodules  and  beds,   see  Gruner,  Bull.  Soc. 
Geol.  France,  xxviii.  (2d  ser.),  p.  62.     Martin,  op.  cit.  iii.  (3d  ser.),  p.  273. 

139  Logan's  "Geology  of  Canada,"  p.  461. 

140  See  A.  F.  Renard  and  J.  Cornet,  Bull.  Acad.  Roy.  Belgique,  xxi.  (1891), 
p.  126.     A.  Strahan,  Quart.  Journ.  Geol.  Soc,  xlvii.  (1891). 


250  TEXT-BOOK   OF   GEOLOGY 

there.  (See  Book  111.  Part  II.  Section  iii.  §  3.)  In  the 
latter  case,  they  may  be  mingled  with  inorganic  sediment, 
so  as  to  pass  into  carbonaceous  shale. 

Peat — vegetable  matter,  more  or  less  decomposed  and 
chemically  altered,  found  throughout  temperate  climates 
in  boggy  places  where  marshy  plants  grow  and  decay.  It 
varies  from  a  pale  yellow  or  brown  fibrous  substance,  like 
turf  or  compressed  hay,  in  which  the  plant-remains  are 
abundant  and  conspicuous,  to  a  compact  dark  brown  or 
black  material,  resembling  black  clay  when  wet,  and  some 
varieties  of  lignite  when  dried.  The  nature  and  proportions 
of  the  constituent  elements  of  peat,  after  being  dried  at 
100°  C.,  are  illustrated  by  the  analysis  of  an  Irish  example 
which  gave — carbon,  60-48;  hydrogen,  6'10;  oxygen,  32*55; 
nitrogen,  0'88;  while  the  ash  was  3 '30.  There  is  always  a 
large  proportion  of  water  which  cannot  be  driven  off  even 
by  drying  the  peat.  In  the  manufacture  of  compressed  peat 
for  fuel  this  constituent,  which  of  course  lessens  the  value 
of  the  peat  as  compared  with  an  equal  weight  of  coal,  is 
driven  off  to  a  great  extent  by  chopping  the  peat  into  fine 
pieces,  and  thereby  exposing  a  large  surface  to  evaporation. 
The  ash  varies  in  amount  from  less  than  1-00  to  more  than 
65  per  cent,  and  consists  of  sand,  clay,  ferric  oxide,  sul- 
phuric acid,  and  minute  proportions  of  lime,  soda,  potash 
and  magnesia.141  Under  a  pressure  of  6000  atmospheres 
peat  is  converted  into  a  hard,  black,  brilliant  substance 
having  the  physical  aspect  of  coal,  and  showing  no  trace 
of  organic  structure.142 

Lignite  (Brown  Coal) — compact  or  earthy,  compressed  and 
chemically  altered  vegetable  matter,  often  retaining  a  lamel- 
lar or  ligneous  texture,  with  stems  showing  woody  fibre 
crossing  each  other  in  all  directions.  It  varies  from  pale 
brown  or  yellow  to  deep  brown  or  black.  Some  shade  of 
brown  is  the  usual  color,  whence  the  name  Brown  Coal, 
by  which  it  is  often  known.  It  contains  from  55  to  75  per 
cent  of  carbon,  has  a  specific  gravity  of  0*5  to  1P5,  burns 
easily  to  a  light  ash  with  a  sooty  flame  and  a  strong  burned 
smell.  It  occurs  in  beds  chiefly  among  the  Tertiary  strata, 
under  conditions  similar  to  those  in  which  coal  is  found  in 


141  See  Senft's  "Humus-,  Marsch-,  Torf-  und  Limonit-bildungen,"  Leipzig, 
1862.  J.  J.  Friih,  "Ueber  Torf  und  Dopplerit,  Zurich,"  1883,  and  the  various 
memoirs  quoted  postea,  p.  802. 

14i  Spring,  Bull.  Acad.  Roy.  Bruxelles,  xJix.  (1880),  p.  367. 


GEOGNOSY  251 

older  formations.  It  may  be  regarded  as  a  stage  in  the 
alteration  and  mineralization  of  vegetable  matter,  inter- 
mediate between  peat  and  true  coal. 

Coal — a  compact,  usually  brittle,  velvet-black  to  pitch- 
black,  iron-black,  or  dull,  sometimes  brownish  rock,  with 
a  grayish- black  or  brown  streak,  and  in  some  varieties  a 
distinctly  cubical  cleavage,  in  others  a  conchoidal  fracture. 
It  contains  from  75  to  90  per  cent  of  carbon,  and  a  small 
percentage  of  sulphur,  generally  in  the  form  of  iron-disul- 
phide.  It  has  a  specific  gravity  of  1-2-1  '35,  and  burns  with 
comparative  readiness,  giving  a  clear  flame,  a  strong  aro- 
matic or  bituminous  smell,  some  varieties  fusing  and  caking 
into  cinder,  others  burning  away  to  a  mere  white  or  red 


Ptg.  S6.— Microscopic  Structure  of  Dalkeith  Coal, 

showing  Lycopodiaceous   Sporangia 

(magnified  300  Diameters). 

ash.  Though  it  consists  of  compressed  vegetation,  no  trace 
of  organic  structure  is  usually  apparent.143  An  attentive 
examination,  however,  will  often  disclose  portions  of  stems, 
leaves,  etc.,  or  at  least  of  carbonized  woody  fibre.  Some 
kinds  are  almost  wholly  made  up  of  the  spore-cases  of 
lycopodiaceous  plants  (Fig.  25).  There  is  reason  to  believe 
that  different  varieties  of  coal  may  have  arisen  from  original 
diversities  in  the  nature  of  the  vegetation  out  of  which  they 
were  formed.  The  accompanying  table  shows  the  chemical 
gradation  between  unaltered  vegetation  and  the  more  highly 
mineralized  forms  of  coal. 


143  On  the  influence  of  pressure  on  the  formation  of  coal,  see  Fre'my,  Compt. 
rend.  20th  May  1879.     Spring,  Bull.  Acad.  Roy.  Bruxelles,  1880,  p.  367. 


TEXT-BOOK    OF   GEOLOGY 


TABLE   SHOWING   THE   GRADUAL   CHANGE   IN   COMPOSITION   FBOM   WOOD  TO 
CHARCOAL'" 


a 

i 

§ 

UP 

Substance 

I 

•1 

!°'f° 

° 

I 

O 

fill! 

1.  Wood  (mean  of  several  analyses)  
2.  Peat    (         "          "             "      )  

100 
100 

12-18 
9-85 

83-07 
55-67 

1-80 
2-89 

3    Lignite  (mean  of  15  varieties) 

100 

8-37 

42-42 

3-07 

4.  Tan-yard  coal  of  S.  Staffordshire  basin 

100 

6-12 

21-23 

3-47 

5    Steam  coal  from  the  Tyne                 .... 

100 

5-91 

18-32 

3-62 

6    Peutrefelin  coal  of  S   Wales 

100 

4-75 

5-28 

4-09 

7.  Anthracite  from  Pennsylvania,  TJ.  S... 

100 

2-84 

1-74 

2-63 

Coal  occurs  in  seams  or  beds  intercalated  between  strata 
of  sandstone,  shale,  fireclay,  etc.,  in  geological  formations 
of  Palaeozoic,  Secondary,  and  Tertiary  age.  It  should  be 
remembered  that  the  word  coal  is  rather  a  popular  than 
a  scientific  term,  being  indiscriminately  applied  to  any 
dense,  black  mineral  substance  capable  of  being  used  as 
fuel.  Strictly  employed,  it  ought  only  to  be  used  with 
reference  to  Beds  of  fossilized  vegetation,  the  result  either 
of  the  growth  of  plants  on  the  spot  or  of  the  drifting  of 
them  thither. 

The  following  analyses  show  the  chemical  composition 
of  peat,  lignite,  and  some  of  the  principal  varieties  or  coal:14* 


Peat 

Devon- 
shire 

Lignite 

Bovey, 
Tracey, 
Devon 

Caking 
Coal 
North- 
umber- 
land 

Non-Cak- 
ing Coal 

8.  Staf- 
fordshire 

Cannel 
Coal 

Wigan 

Anthra- 
cite 

S.  Wales 

Carbon  

54-02 

66-31 

78-69 

78-57 

80-07 

90-39 

Hydrogen... 

5-21 

5-63 

6-00 

5-29 

5-53 

3-28 

Oxygen  

28-18 

22-86 

10-07 

12-88 

8-08 

2-98 

Nitrogen  ... 

2-30 

0-57 

2-37 

1-84 

2-12 

0-83 

Sulphur  ... 

0-56 

2-36 

1-51 

0'39 

1-50 

0-91 

Ash          

9-73 

2-27 

1-36 

1-03 

2-70 

1-61 

Specific  gravity  

0-850 

1-129 

1-259 

1-278 

1-276 

1-392 

144  Percy's  "Metallurgy,"  vol.  i.  p.  268. 
146  From  Percy's  "Metallurgy,"  vol.  i. 


GEOGNOSY  253 

These  analyses  are  exclusive  of  water,  which  in  the  peat 
amounted  to  2&-56,  and  in  the  lignite  to  34-66  per  cent. 

Arthracite — the  most  highly  mineralized  form  of  vegetation 
— is  tn  iron-black  to  velvet-black  substance,  with  a  strong 
metahoidal  to  vitreous  lustre,  hard  and  brittle,  containing 
over  90  per  cent  of  carbon,  with  a  specific  gravity  of 
1-35-1-7.  It  kindles  with  difficulty,  and  in  a  strong  draught 
burns  without  fusing,  smoking,  or  smelling,  but  giving  out 
a  great  heat.  It  is  a  coal  from  which  the  bituminous  parts 
have  been  eliminated.  It  occurs  in  beds  like  ordinary  coal, 
but  in  positions  where  probably  it  has  been  subjected  to 
some  change  whereby  its  volatile  constituents  have  been 
expelled.  It  is  found  largely  in  South  Wales,  and  sparingly 
in  the  Scottish  coal-fields  where  the  ordinary  coal-seams 
have  been  approached  by  intrusive  masses  of  igneous  rock. 
It  is  largely  developed  in  the  great  coal-field  of  Pennsylvania. 
Some  Lower  Silurian  shales  are  black  from  diffused  anthracite, 
and  have  in  consequence  led  to  fruitless  searches  for  coal. 

Oil-shale  (Brandschiefer} — shale  containing  such  a  propor- 
tion of  hydrocarbons  as  to  be  capable  of  yielding  mineral 
oil  on  slow  distillation.  This  substance  occurs  as  ordinary 
shales  do,  in  layers  or  beds,  interstratified  with  other  aqueous 
deposits,  as  in  the  Scottish  coal-fields.  It  is  in  a  geological 
sense  true  shale,  and  owes  its  peculiarity  to  the  quantity  of 
vegetable  (or  animal)  matter  which  has  been  preserved 
among  its  inorganic  constituents.  It  consists  of  fissile  argil- 
laceous layers,  highly  impregnated  with  bituminous  matter, 
passing  on  one  side  into  common  shale,  on  the  other  into 
cannel  or  parrot  coal.  The  richer  varieties  yield  from  30 
to  40  gallons  of  crude  oil  to  the  ton  of  shale.  They  may  be 
distinguished  from  non-bituminous  or  feebly  bituminous 
shales  (throughout  the  shale  districts  of  Scotland),  by  the 
peculiarity  that  a  thin  paring  curls  up  in  front  of  the  knife, 
and  shows  a  brown  lustrous  streak.  Some  of  the  oil-shales 
in  the  Lothians  are  crowded  with  the  valves  of  ostracod 
crustaceans,  besides  scales,  coprolites,  etc.,  of  ganoid  fishes. 
It  is  possible  that  the  bituminous  matter  may  in  some  cases 
have  resulted  from  animal  organisms,  though  the  abun- 
dance of  plant  remains  indicates  that  it  is  probably  in  most 
cases  of  vegetable  origin.  Under  the  name  "pyroschists" 
Sterry  Hunt  classed  the  clays  or  shales  (of  ail  geological 
ages)  which  are  hydrocarbonaceous,  and  yield  by  distilla- 
tion volatile  hydrocarbons,  infammable  gas,  etc. 

Petroleum — a  general  term,  under  which  is  included  a  series 
of  natural  mineral  oils.  These  are  fluid  hydrocarbon  com- 


254  TEXT-BOOK   OF   GEOLOGY 

pounds,  varying  from  a  thin,  colorless,  watery  liquidity  to 
a  black,  opaque,  tar-like  viscidity,  and  in  specific  gravity 
from  0-8  to  1-1.  The  paler,  more  limpid  varieties  are  gen- 
erally called  naphtha,  the  darker,  more  viscid  kinds 
mineral  tar,  while  the  name  petroleum,  or  rock-oil, 
has  been  more  generally  applied  to  the  intermediate  kinds. 
Petroleum  occurs  sparingly  in  Europe.  A  few  localities  for 
it  are  known  in  Britain.  It  is  found  in  large  quantity  along 
the  country  stretching  from  the  Carpathians,  through  Gal- 
licia  and  Moldavia,  also  at  Baku  on  the  Caspian.146  The 
most  remarkable  and  abundant  display  of  the  substance, 
however,  is  in  the  so-called  oil-regions'  of  North  America, 
particularly  in  Western  Canada  and  Northern  Pennsylvania, 
where  vast  quantities  of  it  have  been  obtained  in  recent 
years.  In  Pennsylvania  it  is  found  especially  in  certain 
porous  beds  of  sandstone  or  "sand-rocks,"  which  occur  as 
low  down  as  the  Old  Red  Sandstone,  or  even  as  the  top 
of  the  Silurian  system.  In  Canada  it  is  largely  present  in 
still  lower  strata.  Its  origin  in  these  ancient  formations, 
where  it  cannot  be  satisfactorily  connected  with  any  de- 
structive distillation  of  coal,  is  still  an  unsolved  problem. 
Asphalt — a  smooth,  brittle,  pitch-like,  black  or  brownish- 
black  mineral,  having  a  resinous  lustre  and  conchoidal 
fracture,  streak  paler  than  surface  of  fracture,  and  specific 
gravitv  of  1-0  to  1-68.  It  melts  at  about  the  temperature 
of  boiling  water,  and  can  be  easily  kindled,  burning  with 
a  bituminous  odor  and  a  bright  but  smoky  flame.  It  is 
composed  chiefly  of  hydrocarbons,  with  a  variable  admix- 
ture of  oxygen  and  nitrogen.  It  occurs  sometimes  in  asso- 
ciation with  petroleum,  of  which  it  may  be  considered  a 
hardened  oxidized  form,  sometimes  as  an  impregnation 
filling  the  pores  or  chinks  of  rocks,  sometimes  in  inde- 
pendent beds.  In  Britain  it  appears  as  a  product  of  the 
destructive  distillation  of  coals  and  carbonaceous  shales  by 
intrusive  igneous  rocks,  as  at  Binny  Quarry,  Linlithgow- 
shire,  but  also  in  a  number  of  places  where  its  origin  is 
not  evident,  as  in  the  Cornish  and  Derbyshire  mining  dis- 
tricts, and  among  the  dark  flagstones  of  Caithness  and 
Orkney,  which  are  laden  with  fossil  fishes.  At  Seyssel 
(Departement  de  1'Ain)  it  forms  a  deposit  2500  feet  long 
and  800  feet  broad,  which  yields  1500  tons  annually.  It 
exudes, in  a  liquid  form  from  the  ground  round  the  borders 

146  Abich,  Jahrb.  Geol.  Reichsanst,  xxix.  (1879),  p.  165.  Trautschold, 
Zeitsch.  Deutsch.  Geol.  Ges.  xxvi.  (1874),  p.  257.  See  postea,  Book  III. 
Part  I.  Sect.  i.  §  2,  where  other  authorities  are  cited. 


GEOGNOSY  255 

of  the  Dead  Sea.  In  Trinidad  it  forms  a  lake  li  miles  in 
circumference,  which  is  cool  and  solid  near  the  shore,  but 
increases  in  temperature  and  softness  toward  the  centre. 

Graphite. — This  mineral  occurs  in  masses  of  sufficient  size 
and  importance  to  deserve  a  place  in  the  enumeration  of 
carbonaceous  rocks.  Its  mineralogical  characters  have  al- 
ready (p.  124)  been  given.  It  occurs  in  distinct  lenticular 
beds,  and  also  diffused  in  minute  scales,  through  slates, 
schists,  and  limestones  of  the  older  geological  formations,  as 
in  Cumberland,  Scotland,  Canada,  and  Bohemia.  It  is  likewise 
found  occasionally  as  the  result  of  the  alteration  of  a  coal 
seam  by  intrusive  basalt,  as  at  New  Cumnock  in  Ayrshire. 

5.  FERRUGINOUS. — The  decomposition  of  vegetable  mat- 
ter in  marshy  places  and  shallow  lakes  gives  rise  to  certain 
organic  acids,  which,  together  with  the  carbonic  acid  xso 
generally  also  present,  decompose  the  ferruginous  minerals 
of  rocks  and  carry  away  soluble  salts  of  iron.  Exposure 
to  the  air  leads  to  the  rapid  decomposition  and  oxidation 
of  those  solutions,  which  consequently  give  rise  to  precipi- 
tates, consisting  partly  of  insoluble  basic  salts  and  partly 
of  the  hydrated  ferric  oxide.  These  precipitates,  mingled 
with  clay,  sand,  or  other  mechanical  impurity,  and  also 
with  dead  and  decaying  organisms,  form  deposits  of  iron- 
ore.  Operations  of  this  kind  appear  to  have  been  in  prog- 
ress from  a  remote  geological  antiquity.  Hence  ironstones 
with  traces  of  associated  organic  remains  belong  to  many 
different  geological  formations,  and  are  being  formed  still.1*1 

Bog  Iron-Ore  (Lake-ore,  minerai  des  marais,  Sumpferz) — 
a  dark-brown  to  black,  earthy,  but  sometimes  compact  mix- 
ture of  hydrated  peroxide  of  iron,  phosphate  of  iron,  and 
hydrated  oxide  01  manganese,  frequently  with  clay,  sand, 
and  organic  matter.  An  ordinary  specimen  yielded,  per- 
oxide of  iron,  62-59;  oxide  of  manganese,  8-52:  sand,  11-37; 
phosphoric  acid,  1-50;  sulphuric  acid,  traces;  water  and 
organic  matter,  16-02=100-00.  Bog  iron-ore  may  either  be 
formed  in  situ  from  still  water,  or  may  be  laid  down  by  cur- 
rents in  lakes.  Of  the  former  mode  of  formation,  a  familiar 
illustration  is  furnished  by  the  "moor-band  pan"  or  hard 
ferruginous  crust,  which  in  boggy  places  and  on  some  ill- 
drained  land,  forms  at  the  bottom  of  the  soil,  on  the  top 
of  a  stiff  and  tolerably  impervious  subsoil.  Abundant 
bog-iron  or  lake-ore  is  obtained  from  the  bottoms  of  some 
lakes  in  Norway  and  Sweden.  It  forms  everywhere  on  the 

147  See  Senft's  work  already  (p.  260)  cited,  p.  168;  also  postea,  Book  in. 
Part  II.  Sect.  iii. 


256  TEXT-BOOK    OF   GEOLOGY 

shallower  slopes  near  banks  of  reeds,  where  there  is  no  strong 
current  of  water,  occurring  in  granular  concretions  (Bohnerz) 
that  vary  from  the  size  of  grains  of  coarse  gunpowder  up  to 
nodules  6  inches  in  diameter,  and  forming  layers  10  to  200 
yards  long,  5  to  15  yards  broad,  and  8  to  30  inches  thick. 
These  deposits  are  worked  during  winter  by  inserting  per- 
forated iron  shovels  through  holes  cut  in  the  ice;  and  so 
rapidly  do  they  accumulate,  that  instances  are  known 
where,  after  having  been  completely  removed,  the  ore  at 
the  end  of  twenty-six  years  was  found  to  have  gathered 
again  to  a  thickness  of  several  inches.  A  layer  of  loose 
earthy  ochre  10  feet  thick  is  believed  to  have  formed  in 
600  years  on  the  floor  of  the  Lake  Tisken  near  the  old 
copper  mine  of  Falun  in  Sweden.148  According  to  Ehren- 
berg,  the  formation  of  bog-ore  is  due,  not  merely  to  the 
chemical  actions  arising  from  the  decay  of  organic  matter, 
but  to  a  power  possessed  by  diatoms  of  separating  iron  from 
water  and  depositing  it  as  hydrous  peroxide  within  their 
siliceous  framework. 

Aluminous  Yellow  Iron-Ore  is  closely  related  to  tho  fore- 
going. It  is  a  mixture  of  yellow  or  pale  brown,  Irydrated 
peroxide  of  iron,  with  clay  and  sand,  sometimes  wi  h  sili- 
cate of  iron,  hydrated  oxide  of  manganese,  and  carbonate  of 
lime,  and  occurs  in  dull,  usually  pulverulent  grains  and 
nodules.  Occasionally  these  nodules  may  be  observed  to 
consist  of  a  shell  of  harder  material,  within  which  the  yel- 
low oxide  becomes  progressively  softer  toward  the  centre, 
which  is  sometimes  quite  empty.  Such  concretions  are  known 
as  aetites  or  eagle-stones.  This  ore  occurs  in  the  Coal-measures 
of  Saxony  and  Silesia,  also  in  the  Harz,  Baden,  Bavaria,  etc., 
and  among  the  Jurassic  rocks  in  England. 

Clay-Ironstone  (Sphaerosiderite)  has  been  already  (p.  143) 
referred  to.  It  occurs  abundantly  in  nodules  and  beds  in 
the  Carboniferous  system  in  most  parts 
of  Europe.  The  nodules  are  generally 
oval  and  flattened  in  form,  varying  in 
size  from  a  small  bean  up  to  concretions 
a  foot  or  more  in  diameter,  and  with  an 

internal  system  of  radiating  cracks,  often 

Pig.  26-septarian  NO-   filled    with    calcite   (Fig.    26).      In   many 

dule  of  Clay-u-onstone.     ^^     ^     COQtaiQV  irf  the     centre     some 

organic  substance,  such  as  a  coprolite,  fern,  cone,  shell,  or 
fish,  that  has  served  as  a  surface  round  which  the  iron  in 

148  A.  F.  Thoreld,  Geol.  Foren.  Forhand.  Stockholm,  iii.  p.  2.0,  postea,  pp. 
407,  483. 


GEOGNOSY  257 

the  water  and  the  surrounding  mud  could  be  precipitated. 
Seams  of  clay-ironstone  vary  in  thickness  from  mere  paper- 
like  partings  up  to  beds  several  feet  deep.  The  Cleveland 
seam  in  the  middle  Lias  of  Yorkshire  is  about  20  feet  thick. 
In  the  Carboniferous  system  of  Scotland  certain  seams 
known  as  Black-band  contain  from  10  to  52  per  cent  of  coaly 
matter,  and  admit  of  being  calcined  with  the  addition  of 
little  or  no  fuel.  They  are  sometimes  crowded  with  organic 
remains,  especially  lamellibranchs  (Anthracosia,  Anthra- 
comya,  etc.)  and  fishes  (Rhizodus,  Megalichthys,  etc.). 

A  microscopic  examination  of  some  black-bana  iron- 
stones reveals  a  very  perfect  oolitic  structure,  showing  that 
the  iron  has  either  replaced  an  original  calcareous  oolite  or 
has  been  precipitated  in  water  having  such  a  gentle  move- 
ment as  to  keep  the  granules  quietly  rolling  along,  while  / 
their  successive  concentric  layers  of  carbonate  were  being 
deposited.  Mr.  Sorby  has  observed  in  the  Cleveland  iron- 
stones an  abnormal  form  of  oolitic  structure,  and  remarks 
that  one  specimen  bore  evidence  that  the  iron,  mostly  in  the 
form  of  small  crystals  of  the  carbonate,  had  been  introduced 
subsequently  to  the  formation  of  the  rock,  as  it  had  replaced 
some  of  the  aragonite  of  the  inclosed  shells.149 

The  subjoined  analyses  show  the  composition  of  some 
varieties  of  clay-ironstones.160 

Clay  iron-ore         Black-band        Cleveland  ore 
(Coal  measures),  (Carboniferous),          (Lias), 


Peroxide  of  iron  

Yorkshire 
1-45 

Scotland 
2-72 

Yorkshire 
2-86 

Protoxide  of  iron  

36-14 

40-77 

43-02 

Protoxide  of  manganese. 
Alumina  

1-38 
6'74 

'0-40 
6-87 

Lime  

2-70 

0*90 

6-14  (zinc) 

Magnesia..  .     ,.    

2-11 

0-72 

5-21 

Potash    

0*65 

Silica  

...17-37 

10-10 

7-17 

Carbonic  acid  

..     26-57 

26-41 

25-50 

Phosphoric  acid  
Sulphuric  acid  

0-34 
trace 

1-81 

Iron  pyrites  

0-10 

"Water  

1'77 

1-0 

3-48 

Organic  matter  

2-40 

17-38 

0-15 

Percentage  of  iron... 

99-78 
29-12 

100-00 
34-60 

100-61 
35-46 

M»  Address  to  Geol.  Soc.  February,  1879. 

140  See  Percy's  "Metallurgy,"  voL  ii.     Bischof,  "Chem.  und  Phys.  GeoL' 
aupp.  (1871),  p.  65. 


258  TEXT- BOOK   OF   GEOLOGY 

B.  CRYSTALLINE,  INCLUDING  ROCKS  FORMED  FROM  CHEMICAL  PRECIPITATION 

This  division  consists  mainly  of  chemical  deposits,  but 
includes  also  some  which,  originally  formed  of  organic  cal- 
careous de'bris,  have  acquired  a  crystalline  structure.  The 
rocks  included  in  it  occur  as  laminae  and  beds,  usually  inter- 
calated among  clastic  formations,  such  as  sandstone  and 
shale.  Sometimes  they  attain  a  thickness  of  many  thou- 
sand feet,  with  hardly  any  interstratification  of  mechani- 
cally derived  sediment.  They  are  being  formed  abundantly 
at  the  present  time  by  mineral  springs  and  on  the  floor  of 
inland  seas;  while  on  the  bottom  of  lakes  and  of  the  main 
ocean,  calcareous  organic  accumulations  are  in  progress, 
which  will  doubtless  eventually  acquire  a  thoroughly  crys- 
talline structure  like  that  of  many  limestones. 

Ice. — So  large  an  area  of  the  earth's  surface  is  covered 
with  ice,  that  this  substance  deserves  notice  among  geologi- 
cal formations.  Ice  is  commonly  and  conveniently  classified 
in  two  divisions,  snow-ice  and  water-ice,  according  as 
it  results  from  the  compression  and  alternate  melting  and 
freezing  of  fallen  snow,  or  from  the  freezing  of  the  surface 
or  bottom  of  sheets  of  water. 

S  n  o'w-i  c  e  (see  Book  III.  Part  II.  Sect.  ii.  §  5)  is  of 
two  kinds.  1st,  Fallen  snow  on  mountain  slopes  above  the 
snow-line  gradually  assumes  a  granular  structure.  The  lit- 
tle crystalline  needles  and  stars  of  ice  are  melted  and  frozen 
into  rounded  granules  which  form  a  more  or  less  compact 
mass  known  in  Switzerland  as  Neve  or  Firn.  2d,  When  the 
granular  neve  slowly  slides  down  into  the  valleys,  it  ac- 
quires a  more  compact  crystalline  structure  and  becomes 
glacier-ice.  According  to  the  researches  of  F.  Klocke,  gla- 
cier-ice is,  throughout  its  mass,  an  irregular  aggregate  of 
distinct  crystalline  grains,  the  boundaries  of  which  form 
the  minute  capillary  fissures  so  often  described.1"  Its  struc- 
ture thus  closely  corresponds  to  that  of  marble  (p.  263). 

151  Neues  Jahrb.  1881  (i.),  p.  23.  Grad  and  Dupre  (Ann.  Club.  Alp.  Franc. 
(1874)  show  how  the  characteristic  structure  of  glacier-ice  may  be  revealed  by 
allowing  colored  solutions  to  permeate  it. 


GEOGNOSY  259 

Glacier-ice  in  small  fragments  is  white  or  colorless,  and 
often  shows  innumerable  fine  bubbles  of  air,  sometimes  also 
fine  particles  of  mud.  In  larger  masses,  it  has  a  blue  or 
green-blue  tint,  and  displays  a  veined  structure,  consisting 
of  parallel  vertical  veinings  of  white  ice  full  of  air-bubbles, 
and  of  blue  clear  ice  without  air-bubbles.  Snow-ice  is 
formed  above  the  snow-line,  but  may  descend  in  glaciers 
far  below  it.  It  covers  large  areas  of  the  more  lofty  moun- 
tains of  the  globe,  even  in  tropical  regions.  Toward  the 
poles  it  descends  to  the  sea,  where  large  pieces  break  oif 
and  float  away  as  icebergs. 

Water-i'ce  (see  Book  III.  Part  II.  Sect.  ii.  §5)  is 
formed,  1st,  by  the  freezing  of  the  surface  of  fresh  water 
(river-ice,  lake-ice),  or  of  the  sea  (ice-foot,  floe-ice,  pack- 
ice);  this  is  a  compact,  clear,  white  or  greenish  ice.  2d,  by 
the  freezing  of  the  layer  of  water  lying  on  the  bottom  of 
rivers,  or  the  sea  (bottom-ice,  ground-ice,  anchor-ice);  this 
variety  is  more  spongy,  and  often  incloses  mud,  sand  and 
stones.1" 

Rock-Salt  (Sel  gemme,  Steinsalz,  p.  144)  occurs  in  layers 
or  beds  from  less  than  an  inch  to  many  hundred  feet  in 
thickness.  The  salt  deposits  at  Stassfurt",  for  example,  are 
1197  feet  thick,  of  which  the  lowest  beds  comprise  685  feet 
of  pure  rock-salt,  with  thin  layers  of  anhydrite  J-inch  thick 
dividing  the  salt  at  intervals  of  from  one  to  eight  inches. 
Still  more  massive  are  the  accumulations  of  Sperenberg  near 
Berlin,  which  have  been  bored  through  to  a  depth  of  4200 
feet,  and  those  of  Wieliczka  in  Grallicia  which  are  here  and 
there  more  than  4600  feet  thick. 

The  more  insoluble  salts  (notably  gypsum  or  anhydrite) 
are  apt  to  appear  in  the  lower  parts  of  a  saliferous  series. 
Wlien  purest,  rock-salt  is  clear  and  colorless,  but  usually  is 
colored  red  (peroxide  of  iron),  sometimes  green,  or  blue 
(chloride  or  silicate  of  copper).  It  varies  in  structure,  being 
sometimes  beautifully  crystalline  and  giving  a  cubical  cleav- 
age; laminated,  granularr  or  less  frequently  fibrous/  It 
usually  contains  some  admixture  of  clay,  sand,  anhydrite, 
bitumen,  etc.,  and  is  often  mixed  with  chlorides  of  magne- 
sium, calcium,  etc.  In  some  places  it  is  full  of  vesicles  (not 
infrequently  of  cubic  form)  containing  saline  water;  or  it 
abounds  with  minute  cavities  filled  with  hydrogen,  nitro- 

181  On  the  properties  of  ice  with  some  interesting  geological  bearings,  see 
O.  Pettersson,  "Vega-Expeditionens  Vetenskapliga  lakttagelser, "  vol.  ii.  p.  249, 
Stockholm,  1883. 


260  TEXT-BOOK    OF   GEOLOGY 

gen,  carbon-dioxide,  or  with  some  hydrocarbon  gas.  Oc- 
casionally remains  of  minute  forms  of  vegetable  and  animal 
life,  bituminous  wood,  corals,  shells,  crustaceans,  and  fish 
teeth  are  met  with  in  it.  Owing  to  its  ready  solubility,  it  is 
not  found  at  the  surface  in  moist  climates.  It  has  been 
formed  by  the  evaporation  of  very  saline  water  in  inclosed 
basins — a  process  going  on  now  in  many  salt-lakes  (Great 
Salt  Lake  of  Utah,  Dead  Sea),  and  on  the  surface  of  some 
deserts  (Kirgis  Steppe).  In  different  parts  of  the  world, 
deposits  of  salt  have  probably  always  been  in  progress  from 
very  early  geological  times.  Saliferous  formations  of  Ter- 
tiary and  Secondary  age  are  abundant  in  Europe,  while  in 
America  they  occur  even  in  rocks  as  old  as  the  Upper  Si- 
lurian period,  and  among  the  Punjab  Hills  in  still  more 
ancient  strata.163 

Carnallite — a  chloride  of  potassium  and  magnesium  (p.  144). 
It  occurs  in  a  bed  20  to  30  metres  thick  which  overlies  the 
rock-salt  in  the  saliferous  series  of  Stassfurt,  and  has  been 
found  in  other  old  salt  deposits,  as  well  as  among  the  "salt- 
erns" or  "salines"  along  the  Mediterranean  coast  where  the 
water  of  that  inland  sea  is  evaporated  in  the  manufacture  of 
salt.  It  so  closely  resembles  rock-salt  that  it  was  formerly 
included  with  it,  but  it  is  much  less  frequently  met  with. 
It  is  a  valuable  source  for  the  manufacture  of  potash-salts. 

Limestone  (Calcaire,  Kalkstein) — essentially  a  mass  of  cal- 
cium-carbonate, sometimes  nearly  pure,  and  entirely  or  al- 
most entirely  soluble  in  hydrochloric  acid,  sometimes  loaded 
with  sand,  clay,  or  other  intermixture.  Few  rocks  vary 
more  in  texture  and  composition.  It  may  be  a  hard,  close- 
grained  mass,  breaking  with  a  splintery  or  conchoidal  frac- 
ture; or  a  crystalline  rock  built  up  of  fine  crystalline  grains 
of  calcite,  and  resembling  loaf-sugar  in  color  and  texture; 
or  a  dull  earthy  friable  chalk-like  deposit;  or  a  compact, 
massive,  finely-granular  rock  resembling  a  close-grained 
sandstone  or  freestone.  As  its  hardness  is  about  3,  it  can 
easily  be  scratched  with  a  knife  and  the  white  powder  gives 
a  copious  effervescence  with  acid.  The  specific  gravity 
naturally  varies  according  to  the  impurity  of  the  rock, 
ranging  from  2 -5  to  2-8.  The  colors,  too,  vary  extensively, 
the  most  common  being  shades  of  blue-gray  and  cream-color 
passing  into  white.  Some  limestones  are  highly  siliceous, 
the  calcareous  matter  having  been  accompanied  with  silica 

163  On  salt  deposits  of  various  ages,  see  A.  C.  Ramsay,  Brit.  Assoc.   Rep. 
1880,  p.  10;  also  Index,  sub  voc.  "Salt  Deposits." 


GEOGNOSY  261 

in  the  act  of  deposition;  others  are  argillaceous,  sandy,  fer- 
ruginous, dolomitic,  or  bituminous.  By  far  the  larger  num- 
ber of  limestones  are  of  organic  origin;  though  owing  to 
internal  rearrangement,  their  original  clastic  character  has 
frequently  been  changed  into  a  crystalline  one.  Under  the 
present  subdivision  are  placed  all  those  limestones  which 
have  had  a  distinctly  chemical  origin,  and  also  those  which 
though  doubtless,  in  many  cases,  originally  formed  of  or- 
ganic debris,  have  lost  their  fragmental,  and  have  assumed 
instead  a  crystalline  structure.  (For  the  organic  limestones 
see  p.  244.)" 

Compact,  common  limestone  —  a  fine-grained 
crystalline-granular  aggregate,  occurring  in  beds  or  lamina 
interstratified  with  other  aqueous  deposits.  When  purest  it 
is  readily  soluble  in  acid  with  effervescence,  leaving  little 
or  no  residue.  Many  varieties  occur,  to  some  of  which 
separate  names  are  given.  Hydraulic  limestone  contains  10 
per  cent  or  more  of  silica  (and  usually  alumina)  and,  when 
burned  and  subsequently  mixed  with  water,  forms  a  cement 
or  mortar,  which  has  the  property  of  "setting"  or  harden- 
ing under  water.  Limestones  containing  perhaps  as  much 
as  25  per  cent  of  silica,  alumina,  iron,  etc.,  that  in  them- 
selves would  be  unsuitable  for  many  of  the  ordinary  pur- 
poses for  which  limestones  are  used,  can  be  employed  for 
making  hydraulic  mortar.  These  limestones  occur  in  beds 
like  those  in  the  Lias  of  Lyme  Regis,  or  in  nodules  like 
those  of  Sheppey,  from  which  Roman  cement  is  made. 
Gementstone  is  the  name  given  to  many  pale  dull  ferrugi- 
nous limestones,  which  contain  an  admixture  of  clay,  and 
some  of  which  can  be  profitably  used  for  making  hydraulic 
mortar  or  cement.  Fetid  limestone  (stinkstein,  swinestone) 
gives  off  a  fetid  smell  (sulphuretted  hydrogen  gas),  when 
struck  with  a  hammer.  In  some  cases,  the  rock  seems  to 
have  been  deposited  by  volcanic  springs  containing  decom- 
posable sulphides  as  well  as  lime.  In  other  instances.,  the 
odor  may  be  connected  with  the  decomposition  of  imbedded 
organic  'matter.  In  some  quarries  in  the  Carboniferous 
Limestone  of  Ireland,  as  mentioned  by  Jukes,  the  freshly- 
broken  rock  may  be  smelled  at  a  distance  of  a  hundred  yards 
when  the  men  are  at  work,  and  occasionally  the  stench  be- 
comes so  strong  that  the  workmen  are  sickened  by  it,  and 
require  to  leave  off  work  for  a  time.  Cornstone  is  an  arena- 
ceous or  siliceous  limestone  particularly  characteristic  of 
some  of  the  Palaeozoic  red  sandstone  formations.  Rotten- 
stone  is  a  decomposed  siliceous  limestone  from  which  most 


262  TEXT-BOOK   OF   GEOLOGY 

or  all  of  the  lime  has  been  removed,  leaving  a  siliceous 
skeleton  of  the  rock.  A  similar  decomposition  takes  place 
in  some  ferruginous  limestones,  with  the  result  of  leaving  a 
yellow  skeleton  of  ochre.  Common  limestone,  having  been 
deposited  in  water  usually  containing  other  substances  in 
suspension  or  solution,  is  almost  always  mixed  with  impuri- 
ties, and  where  the  mixture  is  sufficiently  distinct  it  receives 
a  special  name,  such  as  siliceous  limestone,  sandy  limestone, 
argillaceous  limestone,  bituminous  limestone,  dolomitic  lime- 
stone. 

Travertine  (calcareous  tufa,  calc-sinter)  is  the  porous 
material  deposited  by  calcareous  springs,  usually  white  or 
yellowish,  varying  in  texture  from  a  soft  chalk-like  or  marly 
substance  to  a  compact  building-stone.  (See  Book  III.  Part 
II.  Sect.  iii.  §§  3,  6.)  Stalactite  is  'the  name  given  to  the 
calcareous  pendent  deposit  formed  on  the  roofs  of  limestone- 
caverns,  vaults,  bridges,  etc. ;  while  the  water,  from  which 
the  hanging  lime-icicles  are  derived,  drips  to  the  floor,  and 
on  further  evaporation  there,  gives  rise  to  the  crust-like 
deposit  known  as  stalagmite.  Mr.  Sorby  has  shown  that  in 
the  calcareous  deposits  from  fresh  water  there  is  a  constant 
tendency  toward  the  production  of  calcite  crystals  with  the 
principal  axis  perpendicular  to  the  surface  of  deposit. 
Where  that  surface  is  curved,  there  is  a  radiation  or  con- 
vergence of  the  fibre-like  crystals,  well  seen  in  sections  of 
stalactites  and  of  some  calcareous  tufas  (Fig.  108). 

O  o  1  i  t  e — a  limestone  formed  wholly  or  in  part  of  more 
or  less  perfectly  spherical  grains,  and  having  somewhat  the 
aspect  of  fish-roe.  Each  grain  consists  of  successive  con- 
centric shells  of  carbonate  of  lime,  frequently  with  an  in- 
ternal radiating  fibrous  structure,  which  gives  a  black  cross 
between  crossed  Nicols  (Fig.  27).  The  calcareous  material 
was  deposited  round  some  minute  particle  of  sand  or  other 
foreign  body  which  was  kept  in  motion,  so  that  all  sides 
could  in  turn  become  incrusted  Oolitic  grains  of  this  char- 
acter are  now  forming  in  the  springs  of  Carlsbad  (Sprudel- 
stein);  but  they  may  no  doubt  also  be  produced  where 
gentle  currents  in  lakes,  or  in  partially  inclosed  areas  of 
the  sea,  keep  grains  of  sand  or  fragments  of  shells  drifting 
along  in  water,  which  is  so  charged  with  lime  as  to  be  ready 
to  deposit  it  upon  any  suitable  surface.  An  oolitic  lime- 
stone may  contain  much  impurity.  Where  the  calcareous 
granules  are  cemented  in  a  somewhat  argillaceous  matrix 
the  rock  is  known  in  Germany  as  Rogenstein.  Where  the 
individual  grains  of  an  oolitic  limestone  are  as  large  as 


GEOGNOSY  263 

peas,  the  rock  is  called  a  p  i  s  o  1  i  t  e  (pea-grit).  The  gran- 
ules sometimes  consist  of  aragonite.  Oolitic  structure  is 
found  in  limestones  of  all  ages  from  Palaeozoic  down  to 
recent  times.154  Mr.  E.  Wethered  has  recently  pointed  out 
that  many  oolitic  grains  show  curious  vermiform  twistings 
in  their  outer  concentric  coats,  which  he  regards  as  of 
organic  origin,  either  plant  or  animal  (Q-irvanelld).1™  In 
some  instances  oolites  nave  had  their  calcareous  matter  re- 
placed by  carbonate  or  oxide  of  iron,  so  as  to  become  oolitic 
ironstones. 

Marble  (granular  limestone) — a  crystalline-granular  ag- 
gregate composed  of  crystalline  calcite-granules  of  remark- 
ably uniform  size,  each'  of  which  has  its  own  independent 


Big.  27. — Microscopic  Structure  of  Oolitic     Pig.  28. — Microscopic  Structure  of  white 
Limestone,   after   Sorby.    (Magnified  Statuary  Marble.    (Magnified  50  Di- 

30  Diameters.)  ameters.) 

twin  lamella  ("often  giving  interference  colors)  and  cleavage 
lines.  This  characteristic  structure  is  well  displayed  when 
a  thin  slice  of  ordinary  statuary  marble  is  placed  under  the 
microscope  (Fig.  28).  Typical  marble  is  white,  but  the  rock 
is  also  yellow,  gray,  blue,  green,  red,  black,  or  streaked  and 
mottled,  as  may  be  seen  in  the  numerous  kinds  used  for 
ornamental  purposes.  Its  granular  structure  gives  it  a  re- 
semblance to  loaf-sugar,  whence  the  term  "saccliaroid" 
applied  to  it.  Fine  silvery  scales  of  mica  or  talc  may  often 


154  Oolitic  structure  is  found  even  among  the  limestones  of  the  Dalradian 
metamorphic  series  of  Scotland  (Islay)  which  may  possibly  be  pre- Palaeozoic. 

165  Geol.  Mag.  1889,  p.  196;  Quart.  Journ.  Geol.  Soc.  xlvi.  (1890),  p.  270. 
Mr.  0.  Reid  has  suggested  that  these  tubular  bodies  may  be  due  to  the  deposit 
of  lime  round  organic  filaments  (Algae)  like  the  calcareous  incrustation  formed 
round  fibres  of  hemp  in  kettles  and  boilers. 


264  TEXT-BOOK    OF    GEOLOGY 

be  noticed  even  in  the  purest  marble  (Oipolino).  Some 
crystalline  limestones  associated  with  gneiss  and  schist 
are  peculiarly  rich  in  minerals — mica,  garnet,  tremolite, 
actinolite,  anthophyllite,  zoisite,  vesuvianite,  pyroxenes, 
and  many  other  species  occurring  there  often  in  great 
abundance.  These  inclusions  can  be  isolated  by  dissolv- 
ing the  surrounding  rock  in  acid  (ante,  p.  157). 

Marble  is  regarded  by  most  geologists  as  a  metamorphio 
rock,  that  is,  one  in  which  the  calcium-carbonate,  whether 
derived  from  an  organic  or  inorganic  source,  has  been  en- 
tirely recrystallized  in  situ.  In  the  course  of  this  change 
the  original  clay,  sand  or  other  impurities  of  the  rock  have 
been  also  crystallized,  and  now  appear  as  the  crystalline 
silicates  just  referred  to.  Marble  occurs  in  beds  and  large 
lenticular  masses  associated  with  crystalline  schists  on  many 
different  geological  horizons.  In  Canada  it  occurs  of  Lau- 
rentian;  in  Scotland  of  Cambrian;  in  Utah  of  Upper  Car- 
boniferous; in  Southern  Europe  of  Triassic,  Jurassic  and 
Cretaceous  age. 

Dolomite  (Magnesian  Limestone)  consists  typically  of  a 
yellow  or  white,  crystalline,  massive  aggregate  of  the 
mineral  dolomite;  but  the  relative  proportions  of  the  cal- 
cium and  magnesium-carbonates  vary  indefinitely,  so  that 
every  gradation  can  be  found,  from  pure  limestone  without 
magnesium -carbonate  up  to  pure  dolomite  containing  45'65 
per  cent  of  that  carbonate.  Ferrous  carbonate  is  also  of 
common  occurrence  in  this  rock.  The  texture  of  dolomite 
is  usually  distinctly  crystalline,  the  individual  crystals 
being  occasionally  so  loosely  held  together  that  the  rock 
readily  crumbles  into  a  crystalline  sand.  A  fissured  cavern- 
ous structure  apparently  due  to  a  process  of  contraction 
during  the  process  of  dolomitization,"  is  of  common  oc- 
currence: even  in  compact  varieties,  cellular  spaces  occur, 
lined  with  crystallized  dolomite  (Rauchwacke),  the  crystals 
of  which  are  often  hollow  and  sometimes  inclose  a  kernel  of 
calcite.  Other  varieties  are  built  up  of  spherical,  botryoidal 
and  irregularly-shaped  concretionary  masses.  Dolomite,  in 
its  more  typical  forms,  is  distinguishable  from  limestone 
by  its  greater  hardness  (3-5-4*5),  higher  specific  gravity 
(2*8-2'95),  and  much  less  easy  solubility  in  acid.  It  oc- 
curs sometimes  in  beds  of  original  deposit,  associated  with 
gypsum,  rock-salt  and  other  results  of  the  evaporation  of 
saturated  saline  waters;  it  is  also  found  replacing  what  wag 
once  ordinary  limestone.  The  process  by  which  carbonate 
of  lime  is  replaced  by  carbonate  of  magnesia,  is  referred  to 


GEOGNOSY  265 

in  Book  III.  Part  I.  Sect.  iv.  §  2.IM  Dolomite  sometimes 
forms  picturesque  mountain  masses,  as  in  the  Dolomite 
Mountains  of  the  Eastern  Alps. 

Gypsum — a  fine  granular  to  compact,  sometimes  fibrous 
or  sparry  aggregate  of  the  mineral  gypsum,  having  a  hard- 
ness of  only  1-5-2  (therefore  scratched  with  the  nail),  and 
a  specific  gravity  of  about  2-32,  and  being  unaffected  by 
acids;  hence  readily  distinguishable  from  limestone,  which 
it  occasionally  resembles.  It  is  normally  white,  but  may  be 
colored  gray  or  brown  by  an  admixture  of  clay  or  bitumen, 
or  yellow  and  red  by  being  stained  with  iron-oxide.  It  oc- 
curs in  beds,  lenticular  intercalations  and  strings,  usually 
associated  with  beds  of  red  clay,  rock-salt,  or  anhydrite,  in 
formations  of  many  various  geological  periods  from  Silurian 
(New  York)  down  to  recent  times.  The  Triassic  gypsum 
deposits  of  Thuringia,  Hanover  and  the  Harz  have  long 
been  famous.  One  of  them  runs  along  the  south  flank  of 
the  Harz  Mountains  as  a  great  band  six  miles  long  and 
reaching  a  height  of  sometimes  430  feet. 

Gypsum  furnishes  a  good  illustration  of  the  many  dif- 
ferent ways  in  which  some  mineral  substances  can  originate. 
Thus  it  may  be  produced,"  1st,  as  a  chemical  precipitate 
from  solution  in  water,  as  when  sea- water  is  evaporated; 
2d,  through  the  decomposition  of  sulphides  and  the  action 
of  the  resultant  sulphuric  acid  upon  limestone;  3d,  through 
the  mutual  decomposition  of  carbonate  of  lime  and  sulphates 
of  iron,  copper,  magnesia,  etc. ;  4th,  through  the  hydration 
of  anhydrite;  5th,  through  the  action  of  the  sulphurous 
vapors  and  solutions  of  volcanic  orifices  upon  limestone 
and  calcareous  rocks.1"  It  is  in  the  first  of  these  ways  that 
the  thick  beds  of  gypsum  associated  with  rock-salt  in  many 
geological  formations  have  been  formed.  The  first  mineral 
to  appear  in  the  evaporation  of  sea-water  being  gypsum,  it 
has  been  precipitated  on  the  floors  of  inland  seas  and  saline 
lakes  before  the  more  soluble  salts. 

Anhydrite — the  anhydrous  variety  of  calcium-sulphate, 
occurs  as  a  compact  or  granular,  white,  gray,  bluish  or 
reddish  aggregate  in  saliferous  deposits.  It  is  less  frequent 
than  gypsum,  from  which  it  is  distinguished  by  its  much 
greater  hardness  (3-3 €5)  and  into  which  it  readily  passes  by 


156  On  the  mineralogical  nature  of  dolomite  see  0.  Meyer,  Z.  Deutsch,  GeoL 
Ges.  xxxi.  p.  445,  Loretz,  op.  cit.  xxx.  p.  387,  xxxi.  p.  756.  Renard,  Bull.  Acad. 
Roy.  Belg.  xlvii.  (1879),  No.  5. 

151  Roth.  Chem.  Geol.  i.  p.  553. 
GEOLOGY— Vol.  XXIX— 12 


266  TEXT-BOOK   OF   GEOLOGY 

taking  up  0-2625  of  its  weight  of  water.158  It  often  occurs 
in  thin  seams  or  partings  in  rock-salt;  but  it  also  forms 
large  hill-like  masses,  of  which  the  external  parts  have 
been  converted  into  gypsum. 

Ironstone. — Under  this  general  term  are  included  various 
iron-ores  in  which  the  peroxide,  protoxide,  carbonate,  etc., 
are  mingled  with  clay  and  other  impurities.  They  have 
generally  been  deposited  as  chemical  precipitates  on  the 
bottoms  of  lakes,  under  marshy  ground,  or  within  fissures 
and  cavities  of  rocks.  Some  iron-ores  are  associated  with 
schistose  and  massive  rocks;  others  are  found  with  sand- 
stones, shales,  limestones  and  coals;  while  some  occur  in 
the  form  of  mineral  veins.  Those  which  have  resulted 
from  the  co-operation  of  organic  agencies  are  described  at 
p.  254. 

Haematite  (red  iron-oreX  a  compact,  fine-grained, 
earthy,  or  fibrous  rock  of  a  blood-red  to  brown-red  color, 
but  where  most  crystalline,  steel-gray  and  splendent,  with 
a  distinct  cherry-red  streak.  Consists  of  anhydrous  ferric 
oxide,  but  usually  is  mixed  with  clay,  sand,  or  other  in- 
gredient, in  such  varying  proportions  as  to  pass,  by  insen- 
sible gradations,  into  ferruginous  clays,  sands,  quartz,  or 
jasper.  Occurs  as  beds,  huge  concretionary  masses,  and 
veins  traversing  crystalline  rocks;  sometimes,  as  in  West- 
moreland, filling  up  cavernous  spaces  in  limestone.  Is 
found  occasionally  in  beds  of  an  oolitic  structure  among 
stratified  formations.  Some  at  least  of  the  oolitic  or  piso- 
litic  ironstones  have  resulted  from  the  conversion  of  original 
grains  of  calcite  in  ordinary  oolites  into  carbonate  of  iron 
which  on  oxidation  has  become  magnetite,  haematite,  or 
limonite. 

Limonit^e  (brown  iron-ore),  an  earthy  or  ochreous, 
compact,  fine-grained  or  fibrous  rock,  of  an  ochre-yellow 
to  a  dark  brown  color,  distinguishable  from  haematite  by 
being  hydrous  and  giving  a  yellow  streak.  Occurs  in  beds 
and  veins,  sometimes  as  the  result  of  the  oxidation  of  fer- 
rous carbonate;  abundant  on  the  floors  of  some  lakes;  com 
monly  found  under  marshy  soil  where  it  forms  a  hard 
brown  crust  upon  the  impervious  subsoil  (bog -iron-ore). 
Found  likewise  in  oolitic  concretions  sometimes  as  large  as 
walnuts,  consisting  of  concentric  layers  of  impure  limonite 

158  See  G.  Rose  on  formation  of  this  rock  in  presence  of  a  solution  of  chlo- 
ride of  sodium.  Neues.  Jahrb.  Min.  1871,  p.  932.  Also  Bischof,  "Chem.  und 
Pliya.  Geol."  Suppl.  (1871),  p.  188. 


GEOGNOSY  267 

with  sand  and  clay  (Bohnerz).     (See  p.  255  and  Book  III. 
Part  II.  Sect.  iii.  §  3.) 

Spathic  Iro n-o r e,  a  coarse  or  fine  crystalline  or  dull 
compact  aggregate  of  the  mineral  siderite  or  ferrous  car- 
bonate, usually  with  carbonates  of  calcium,  manganese  and 
magnesium;  has  a  prevalent  yellowish  or  brownish  color, 
and  when  fresh,  its  rhombohedral  cleavage-faces  show  a 

§  early  lustre,  which  soon  disappears  as  the  surface  is  oxi- 
ized  into  limonite  or  hsematite.     Occurs  in  beds  and  veins, 
especially  among  older  geological  formations.     The  colossal 
Erzberg  at  Eisenerz  in  Styria,  which  rises  more  than  2700 
feet  above  the  valley,  consists  almost  wholly  of  siderite.1*9 

Clay-ironstone  (Sphserosiderite),  a.  dull  brown  or 
black,  compact  form  of  siderite,  with  a  variable  mixture 
of  clay,  and  usually  also  of  organic  matter.  Occurs  in  the 
Carboniferous  and  other  formations,  in  the  form  either  of 
nodules,  where  it  has  usually  been  deposited  round  some 
organic  centre,  or  of  beds  interstratified  with  shales  and 
coals.  It  is  more  properly  described  at  p.  256,  with  the 
organically  derived  rocks. 

Magnet ic  iro n-o r e,  a  granular  to  compact  aggregate 
of  magnetite,  of  a  black  color  and  streak,  more  or  less  per- 
fect metallic  lustre,  and  strong  magnetism.  Commonly  con- 
tains admixtures  of  other  minerals,  notably  of  hematite, 
chrome-iron,  titanic-iron,  pyrites,  chlorite,  quartz,  horn- 
blende, garnet,  epidote,  felspar.  Occurs  in  beds  and  enor- 
mous lenticular  masses  (Stocke)  among  crystalline  schists, 
likewise  in  segregation-veins  of  gabbros  and  other  eruptive 
rocks;  also  occasionally  in  an  oolitic  form  (probably  as 
a  pseudomorph  after  an  original  calcareous  oolite)  among 
Palaeozoic  rocks,  as  in  the  so-called  "pisolitic  iron-ore"  01 
North  Wales.  Among  the  Scandinavian  gneisses  lies  the 
iron  mountain  of  Gellivara  in  Lulea-Lappmark,  17,000  feet 
long,  8500  feet  broad,  and  525  feet  high. 

Siliceous  Sinter  (Greyserite,  Kieselsinter),  the  siliceous  de- 
posit made  by  hot  springs,  including  varieties  that  are 
crumbling  and  earthy,  compact  and  flinty,  finely  laminated 
and  shaly,  sometimes  dull  and  opaque,  sometimes  trans- 
lucent, with  pearly  or  waxy  lustre,  and  with  chalcedonic 
alterations  in  the  older  parts.  The  deposit  may  occur  as 
an  incrustation  round  the  orifices  of  eruption,  rising  into 
dome-shaped,  botryoidal,  coralloid,  or  columnar  elevations, 
or  investing  leaves  and  stems  of  plants,  shells,  insects,  etc., 

169  Zirkel,  Lehrb.  i.  p.  345. 


268  TEXT-BOOK    OF   GEOLOGY 

or  hanging  in  pendent  stalactites  from  cavernous  spaces 
which  are  from  time  to  time  reached  by  the  hot  water. 
Wben  purest,  it  is  of  snowy  whiteness,  but  is  often  tinted 
yellow  or  flesh  color.  It  consists  of  silica  84  to  91  per  cent, 
with  small  proportions  of  alumina,  ferric  oxide,  lime,  mag- 
nesia, and  alkali,  and  from  5  to  8  per  cent  of  water.  (See 
Book  III.  Part  II.  Sect.  iii.  §  3,  par.  6.) 

Flint  and  Chert  have  been  already  described  among 
the  rocks  of  organic  origin  (ante,  p.  247).  Hornstone,  an  ex- 
cessively compact  siliceous  rock,  usually  of  some  dull  dark 
tint,  occurs  in  nodular  masses  or  irregular  bands  and  veins. 
The  name  has  sometimes  been  applied  to  fine  flinty  forms 
of  felsite.  Vein-Quartz  may  be  alluded  to  here  as  a  substance 
which  sometimes  occurs  in  large  masses.  It  is  a  massive 
form  of  quartz  found  filling  veins  (sometimes  many  yards 
broad)  in  crystalline  and  clastic  rocks;  more  especially  in 
metamorphic  areas.  (See  Quartz  Bocks,  p.  310.) 

Some  of  the  other  varieties  of  silica  occurring  in  large 
masses  may  be  classed  as  rocks.  Such  are  Jasper,  and 
Ferruginous  Quartz.  These,  as  well  as  common 
vein-quartz,  occur  as  veins  traversing  both  stratified  and 
unstratified  rocks;  also  as  beds  associated  with  the  crystal 
line  schists.  With  them  may  be  grouped  Lydian-Stone 
(Lydite,  Kieselschiefer\  a  black  or  dark-colored,  excessively 
compact,  hard,  infusible  rock  with  splintery  fracture,  occur- 
ing  in  thin,  sharply  defined  bands,  split  by  cross  joints  into 
polygonal  fragments,  which  are  sometimes  cemented  by  fine 
layers  of  quartz.  It  consists  of  an  intimate  mixture  of  silica 
with  alumina,  carbonaceous  materials,  and  oxide  of  iron, 
and  under  the  microscope  shows  minute  quartz-granules 
with  dark  amorphous  matter.  It  occurs  in  thin  layers  or 
bands  in  the  Silurian  and  later  Palaeozoic  formations  inter- 
stratified  with  ordinary  sandy  and  argillaceous  strata.  As 
these  rocks  have  not  oeen  materially  altered,  the  bands  of 
Lydian-stone  may  be  of  original  formation,  though  the  ex- 
tent to  which  they  are  often  veined  with  quartz  shows  that 
they  have,  in  many  cases,  been  permeated  by  siliceous  water 
since  their  deposit.  The  siliceous  rocks  due  to  the  opera- 
tions of  plant  and  animal  life  are  described  on  p.  247,  also  in 
Book  III.  Part  II.  Sect.  iii.  §  3. 

Some  originally  clastic  siliceous  rocks  have  acquired  a 
more  or  less  crystalline  structure  from  the  action  of  thermal 
water  or  otherwise.  One  of  the  most  marked  varieties  has 
been  termed  Crystallized  Sandstone  (see  p.  232).  Another 
variety,  known  as  Quartzite,  is  a  granular  and  compact  ag- 


GEOGNOSY  269 

gregate  of  quartz,  which  will  be  described  in  connection  with 
the  schistose  rocks  among  which  it  generally  occurs  (p.  311). 

II.  MASSIVE — ERUPTIVE — IGNEOUS 

Almost  all  the  members  of  this  important  subdivision 
have  been  produced  from  within  the  crust  of  the  earth,  in  a 
molten  condition.  Nearly  all  consist  of  two  or  more  min- 
erals. Considered  from  a  chemical  point  of  view,  they  may 
be  described  as  mixtures,  in  different  proportions,  of  sili- 
cates of  alumina,  magnesia,  lime,  potash,  and  soda,  usually 
with  magnetic  iron  and  phosphate  of  lime.  In  one  series, 
the  silicic  acid  has  not  been  more  than  enough  to  combine 
with  the  different  bases;  in  another,  it  occurs  in  excess  as 
free  quartz.  Taking  this  feature  as  a  basis  of  arrangement, 
some  petrographers  have  proposed  to  divide  the  rocks  into 
an  acid  group,  including  such  rocks  as  granite,  quartz-por- 
phyry and  rhyolite,  where  the  percentage  of  silica  ranges 
from  60  to  75  or  more,  a  basic  group,  typified  by  such  rocks 
as  basalt,  where  the  proportion  of  silica  is  only  about  50  per 
cent  or  less,  and  an  intermediate  group  represented  by  the 
andesites  with  a  proportion  of  silica  ranging  between  that  of 
the  other  two  groups.  Ib° 

In  the  vast  majority  of  igneous  rocks,  the  chief  silicate 
is  a  felspar — the  number  of  rocks  where  the  felspar  is  repre- 
sented by  another  silicate  (as  leucite  or  nepheline)  being 
comparatively  few  and  unimportant.  As  the  felspars  group 
themselves  into  two  divisions,  the  monoclinic  or  orthoclase, 
and  the  triclinic  or  plagioclase,  the  former  with,  on  the 
whole,  a  preponderance  of  silica;  and  as  these  minerals  oc- 


160  See  a  paper  on  the  chemical  relations  of  the  eruptive  rocks  by  Prof. 
Rosenbusch,  Tschermak's  Min.  Mittheil.  xi.  (1889),  p.  144,  also  the  paper 
quoted  in  footnote  (163)  on  p.  272,  and  a  Memoir  OH  "the  origin  of  Igneous 
Rocks,"  by  J.  P.  Iddings,  Phil.  Soc.  Washington,  1892,  p.  90. 


270  TEXT-BOOK    OF   GEOLOGY 

cur  under  tolerably  distinct  and  definite  conditions,  other 
petrographers  divide  the  felspar-bearing  Massive  rocks  into 
two  series:  (1)  the  Orthoclase  rocks,  having  orthoclase  as 
their  chief  silicate,  and  often  with  free  silica  in  excess,  and 
(2)  the  Plagioclase  rocks,  where  the  chief  silicate  is  some 
species  of  triclinic  felspar.  The  former  series  corresponds 
generally  to  the  acid  group  above  mentioned,  while  the 
plagioclase  rocks  are  intermediate  and  basic.  It  has  been 
objected  to  this  arrangement  that  the  so-called  plagioclase 
felspars  are  in  reality  very  distinct  minerals,  with  propor- 
tions of  silica,  ranging  from  43  to  69  per  cent;  soda  from 
0  to  12;  and  lime  from  0  to  20.""  In  addition  to  the  felspar- 
rocks,  there  must  be  noted  those  in  which  felspar  is  either 
wholly  absent  or  sparingly  present,  and  where  the  chief  part 
in  rock-making  has  been  taken  by  nepheline,  leucite,  oli- 
vine,  or  serpentine. 

From  the  point  of  view  of  internal  structure,  a  classifica- 
tion based  upon  microscopic  research  has  been  adopted  by 
other  writers,  who  recognize  three  leading  types  of  micro- 
structure —  Granular,  Porphyritic  and  Glassy,  or  Holocrys- 
lalline,  Hemi-crystalline  and  Vitreous.  MM.  Fouque  and 
Michel-Levy,  pointing  out  that  most  eruptive  rocks  are  the 
result  of  successive  stages  of  crystallization,  each  recogniz- 
able by  its  own  characters,  show  that  two  phases  of  consoli- 
dation are  specially  to  be  observed,  the  first  (porphyritic) 
marked  by  the  formation  of  large  crystals  (phenocrysts), 
which  were  often  broken  and  corroded  by  mechanical  and 
chemical  action  within  the  still  unsolidified  magma;  the 
second  by  the  formation  of  smaller  crystals,  crystallites, 
etc.,  which  are  molded  round  the  older  series.  In  some 

141  Dana,  Amer.  Journ.  Sci.  1878,  p.  432.  The  modern  methods  of  separat- 
ing the  felspars  remove  some  of  the  difficulty  above  referred  to. 


GEOGNOSY  271 

rocks  the  former,  in  others  the  latter  of  these  two  phases 
is  alone  present.  Two  leading  types  of  structure  are  rec- 
ognized by  these  authors  among  the  eruptive  rocks.  1. 
Granitoid,  where  the  constituents  are  mainly  those  of 
the  second  epoch  of  consolidation,  'but  where  neither  amor- 
phous magma  nor  crystallites  are  to  be  seen.  This  struc- 
ture includes  three  varieties,  (a)  the  granitoid  proper,  having 
crystals  of  approximately  equal  size;  (b)  pegmatoid,  where 
there  has  been  a  simultaneous  crystallization  and  regular 
arrangement  of  two  constituents;  (c)  ophitic,  in  which  the 
felspars  are  ranged  parallel  to  one  of  their  crystalline  faces, 
forming  a  kind  of  transition  into  microlitic  rocks.  2. 
Trachytoid,  distinguished  by  a  more  marked  contrast 
between  the  crystals  of  the  first  and  second  consolidation, 
the  usual  presence  of  an  amorphous  magma,  and  the  fluxion 
structure.  Three  varieties  are  named:  (a)  petrosiliceous,  with 
trains  and  spherulites  of  a  finely  clouded  substance  charac- 
teristic of  the  more  acid  rocks;  (b}  microlitic,  characterized 
by  the  abundance  of  microlites  of  felspars  and  other  min- 
erals; (c)  vitreous,  derived  from  the  two  foregoing  varieties 
by  the  predominance  of  the  amorphous  paste.192 

It  is  common  to  introduce  a  chronological  element  into 
the  classification  of  the  massive  rocks  and  to  divide  them 
into  an  ancient  (Palaeozoic  and  Mesozoic)  and  modern  (Ter- 
tiary and  recent)  series.  Certain  broad  distinctions  can 
doubtless  be  made  between  many  ancient  and  modern 
eruptive  rocks;  but,  for  reasons  already  stated,  it  seems 
inexpedient,  in  the  present  state  of  our  knowledge,  to  em- 
ploy relative  antiquity  (which  must  be  determined  by  a 
totally  distinct  branch  of  geological  inquiry,  and  may  be 

1M  "Mineralogie  Micrographique, "  p.  150. 


272  TEXT-BOOK    OF   GEOLOGY 

erroneously    determined)  as    a  basis   of   petrographical   ar- 
rangement.19* 

In  the  following  arrangement  the  threefold  division  first 
mentioned  above  is  adopted,  according  to  the  relative  abun- 
dance of  silica:  1st,  Acid;  2d,  Intermediate;  8d,  Basic.  In 
each  of  these  series  there  is  a  range  of  structure  from  com- 
pletely crystalline  to  completely  glassy.  The  holocrystal- 
line  rocks  are  as  a  rule  the  deep-seated  representatives  of 
each  series,  while  the  vitreous  and  semi-vitreous  are  those 
which  have  either  been  erupted  to  the  surface  or  have  been 
connected  with  volcanic  rather  than  plutonic  action.  No 
system  of  classification  yet  proposed  can  avoid  incongruities, 
and  it  must  be  remembered  that  the  hard  and  fast  lines  of 
our  nomenclature  do  not  represent  any  really  abrupt  demar- 
cations in  nature.  As  one  rock  graduates  into  another,  our 
terminology  should  be  elastic,  so  as  to  include  such  transi- 
tional forms. 

i.  Acid  Series 

In  this  family  the  silicic  acid  has  been  in  such  excess  as 
often  to  separate  out  in  the  form  of  free  quartz.  Sometimes, 
as  in  granite,  it  has  not  assumed  a  definitely  crystallized  form, 
but  is  molded  round  the  other  crystals  as  a  later  stage  of 
consolidation.  In  other  rocks  (quartz- porphyry,  etc.)  it  oc- 
curs as  a  product  of  earlier  consolidation,  and  often  assumes 
perfect  crystallographic  contours,  occurring  even  in  double 
pyramids.'  The  texture  of  the  rocks  is  (1>  holocrystalline 
or  crystalline-granular  (granitoid),  as  typically  developed  in 
granite;  (2)  hemi-crystalline  (porphyritic,  trachytoid),  as  in 
quartz-porphyry  or  felsite;  (3)  vitreous,  as  in  obsidian. 

Granite."4— A  thoroughly  crystalline-granular  admixture  of 


1M  For  a  tabular  arrangement  of  the  massive  (eruptive)  rocks  and  critical 
remarks  on  their  classification,  see  Roaenbusch,  Neues  .Tahrb.  1882,  ii.  p.  1. 

184  Oil  the  structure  of  granite,  see  the  manuals  of  Zirkel  and  Rosenbusch 
and  the  memoirs  there  cited;  also  Zirkel's  "Microscop.  Petrography,"  1876, 
p.  39;  Phillips,  Q.  J.  Geol.  Soc.  xxxi.  p.  330;  xxxvi.  p.  1.  J.  C.  Ward,  op.  cit. 
p.  569;  and  xxxii.  p.  1.  King's  "Systematic  Geology"  (vol.  i.  of  Explor.  40th 


GEOGNOSY  278 

quartz,  felspar,  and  mica,  in  particles  of  tolerably  uniform 
size  (Figs.  15  and  29).  The  felspar  is  chiefly  white  or  pink 
orthoclase,  but  triclinic  felspars  (oligpclase  and  albite)  may 
often  be  observed  in  smaller  quantity,  frequently  distin- 
guishable by  their  fine  striation 
and  more  waxy  lustre.  Micro- 
cline  is  not  infrequent,  as  well 
as  the  intercrystallization  of  or- 
thoclase and  plagioclase  (Per- 
thite).  The  mica  may  be  the 
potash  (muscovite)  variety,  usu- 
ally of  a  white  silvery  'aspect, 
but  more  commonly  biotite  or 
other  dark  brown  or  black  vari- 
ety. The  quartz  may  be  observed  . 
to'form  a  kind  of  paste  or  magma 

Wrapping     round     the     Other     in-       pjg.  29.— Holocrystalline  Structure 

gradients.     Only   in   cavities   of  of  Granite  (magnified), 

the  granite  do  the  component  minerals  occur  in  independent 
well-formed  crystals,  and  there  too  the  accessory  minerals 
(beryl,  topaz,  tourmaline,  garnet,  etc.)  are  chiefly  found. 

From  a  microscopic  examination  of  granite,  it  was  for- 
merly inferred  that  the  rock  has  a  thoroughly  crystalline 
structure,  with  no  megascopic  ground -mass,  nor  microscopic 
base  of  any  kind  between  the  crystals  or  crystalline  individ- 
uals. More  recent  and  exhaustive  study  of  the  subject, 
however,  has  led  to  the  conclusion  that  though  nothing  like 
a  vitreous,  or  even  porphyritic,  ground-mass  can  be  de- 
tected, there  is  yet  sometimes  discernible  an  analogous 
kind  of  entirely  crystalline  magma,  in  which  the  crystals 
or  crystalline  debris  of  the  rock  are  imbedded,  and  in  which 
they  are  partially  dissolved.  Having  regard  to  the  relations 
between  this  magma  and  its  inclosed  minerals,  M.  Michel- 
Le'vy  has  observed  that  microscopic  examination  points  to 
a  distinction  between  granites  in  which  the  quartz  is  more 
recent  than  the  other  constituents  and  has  consolidated  at 
once,  and  those  in  which  there  are  remains  of  earlier  bi- 
pyramidal  quartz.  He  distinguishes  these  two  series  as — 
(A)  Ancient  granites,  composed  of  black  mica,  hornblende, 
oligoclase,  and  orthoclase,  forming  a  crystalline  debris  im- 

Parallel),  p.  Ill  et  seq.  Michel-Levy,  Bull.  Soc.  G-eol.  Prance,  3d  ser.  iii.  p.  199. 
Rosenbusch,  Zeitsch.  Deutsch.  Geol.  Gesell.  xxviii.  (1876),  p.  369.  H.  Mohl, 
Nyt.  Mag.  Nat.  xxiii.  p.  1  et  seq.  J.  Lehmann,  "Unterauchungen  iiber  die  Ent- 
stehung  der  Altkry stall inischen  Schiefergesteine,"  1884,  p.  3.  W.  J.  Sollas, 
Trans.  Roy.  Irish  Acad.  xxix.  Part  xiv.  (1891). 


274  TEXT-BOOK    OF   GEOLOGY 

bedded  in  a  more  recent  crystalline  magma  of  orthoclase  and 
quartz.  (B)  Porphyroid  granites,  generally  finer  in  grain 
than  the  preceding,  and  further  distinguished  by  the  oc- 
currence of  bi-pyramidal  crystals  of  quartz  (which  made 
their  appearance'  between  the  old  felspar  and  the  recent 
orthoclase),  and  of  a  notable  quantity  of  white  mica  (rare 
among  the  ancient  granites)  posterior  in  advent  even  to  the 
more  recent  quartz.1" 

Among  the  component  minerals  of  granite,  the  quartz 
presents  special  interest  under  the  microscope.  It  is  often 
found  to  oe  full  of  cavities  containing  liquid,  sometimes 
in  such  numbers  as  to  amount  to  a  thousand  millions  in 
a  cubic  inch  and  to  give  a  milky  turbid  aspect  to  the  min- 
eral. The  liquid  in  these  cavities  appears  usually  to  be 
water  containing  sodium  and  potassium  chlorides,  with 
sulphates  of  these  metals  and  of  calcium  (p.  196). 

The  mean  of  eleven  analyses  of  granites  made  by  Dr. 
Haughton  gave  the  following  average  composition:  silica, 
72-07;  alumina,  14-81;  peroxide  of  iron,  2-22;  potash,  6-11; 
soda,  2-79;  lime,  1-63;  magnesia,  0-33;  loss  by  ignition, 
1-09;  total,  100-05,  with  a  mean  specific  gravity  of  2-66. 

Most  large  masses  of  granite  present  differences  of  tex- 
ture in  different  parts  of  their  area.  Some  of  these  varia- 
tions depend  on  the  relation  of  the  mass  to  the  surrounding 
rocks  (Bk.  IV.  Pt.  VII.).  Others  may  occur  in  any  portion 
of  a  granite  boss,  and  have  been  produced  by  the  circum- 
stances in  which  the  mass  consolidated.  Some  granites  are 
marked  by  the  occurrence  of  the  cavities  above  referred  to 
where  the  individual  minerals  have  had  room  to  assume 
sharply  defined  crystalline  forms.  Many  granites  are  apt 
to  be  traversed  by  veins,  sometimes  due  to  a  segregation 
of  the  surrounding  minerals  in  rents  of  the  original  pasty 
magma,  sometimes  to  a  protrusion  of  a  less  coarsely  crys- 
talline (micro-granitic,  felsitic)  material  into  the  main  rock 
(Fig.  30).  Some  of  the  more  important  of  these  varieties 
are  distinguished  by  special  names.  Thus,  where  the  com- 
ponent quartz  and  felspar  have  crystallized  together  so  as 
to  inclose  each  other  and  assume  a  tendency  to  an  orienta- 
tion of  their  longer  axes  in  one  general  direction,  as  thej 
are  specially  apt  to  do  in  segregation -veins,  the  rock  is 
termed  Pegmatite.186  One  of  the  most  interesting  struc- 

166  Bull.  Soc.  Geol.  Prance,  3d  ser.  iii.  (1875),  p.  199. 

166  For  an  admirable  and  exhaustive  account  of  the  Pegmatite  veins,  and 
their  associated  minerals  in  Southern  Norway,  see  the  great  monograph  by  Prof. 
W.  C.  Brogger  in  Groth's  Zeitsch.  Krystallographie,  xvi.  (1890). 


GEOGNOSY  275 

tural  varieties  is  the  form  of  pegmatite  termed  Graphic 
Granite,  in  which  the  orientation  of  the  quartz  and  fel- 
spar is  singularly  well  developed  (Fig.  31).  The  quartz  has 
assumed  the  shape  of  long  imperfect  columnar  shells,  placed 
parallel  to  each  other  and  inclosed  within  the  orthoclase,  so 
that  a  transverse  section  bears  some  resemblance  to  Hebrew 
writing.  The  two  minerals  have  crystallized  together,  with 
their  principal  axes  parallel.  This  intergrowth  seems  to 
show  that  there  could  have  been  little  or  no  internal  move- 


Fig.  30.— Vein  of  finer  grain  (aplite)  traversing  a  coarsely 
crystalline  Granite. 

ment  of  the  veins,  in  which  it  so  frequently  occurs,  when 
the  component  minerals  assumed  their  crystalline  forms. 
Where  the  intergrowth  is  on  a  minute  scale  it  is  known  as 
micropegmatite,  and  it  forms  the  base  of  the  rock 
to  which  the  name  of  Granop  hy  re  has  been  given  (Fig. 
6).  Here  and  therec  an  example  may  be  found  of  a  eranite 
becoming  fine-grained,  but  containing  large  scattered  felspar 
crystals.  Such  a  rock  may  be  termed  a  porphyritic  granite. 
Some  granites  abound  in  inclosed  crystalline  concretions  or 
fragments.  These  are  sometimes  mere  segregations  of  the 
materials  of  the  granite,  when  they  are  usually  ovoid  in 


276  TEXT-BOOK   OF   GEOLOGY 

form  and  porphyritic  in  structure;  in  other  cases,  they  are 
fragments  of  other  rocks,  and  are  then  commonly  schistose 
in  structure  and  irregular  in  form.167  In  rare  examples  the 
component  minerals  of  granite  have  crystallized  with  a 
radial  concentric  arrangement  into  rounded  ball-like  aggre- 
gates (spheroidal,  orbicular  granite). J68  In  the  centre,  as 
well  as  round  the  edges  of  large  bosses  of  granite,  the  min- 
erals occasionally  assume  a  more  or  less  perfectly  schistose 


Fig.  81.— Graphic  Granite  (nat.  size). 

arrangement.     When  this  takes   place,   the   rock  is  called 
gneissose  or  gneiss  granite.     (See  Book  IV.  Part  VII.) 

Differences  in  the  proportions  or  nature  of  the  component 
minerals  have  likewise  suggested  distinctive  names.  Oi  these 
the  following  are  the  more  important:  Gr  r  an  i  ti  t  e  (biotite 
granite)— a  mixture  of  pink  orthoclase  and  abundant  oligo- 
clase,  with  a  little  quartz,  some  blackish  green  magnesia- 
mica,  and  occasionally  with  hornblende  or  augite.  Horn- 
blend  e-gra  nit  e— a  rock  with  hornblende  added  to  the 
other  normal  constituents  of  granite,  and  usually  poorer  in 
quartz  than  normal  granite.  A  well-known  variety  occurs 
at  Syene  in  Upper  Egypt,  whence  it  was  obtained  anciently 
in  large  blocks  for  obelisks  and  other  architectural  works. 
The  well-known  Egyptian  monoliths  are  made  of  it.  It  was 
called  by  Pliny  "Syenite" — a  name  adopted  by  Werner  as 
a  general  designation  for  horneblendic  granites  without 
quartz.  The  rock  of  Syene  is  really  a  hornblende-biotite- 


'«  J.  A.  Phillips,  Q.  J.  Geol.  Soc.  xxxvi.  (1880),  p.  1. 

188  W.  C.  Brogger  and  H.  Backstrom,  Geol.  Stockholm  Forhandl.  ix.  (1887^ 
p.  37.  Hatch,  Quart..  Journ.  Geol.  Soc.  xliv.  (1888),  p.  648,  and  authorities 
there  cited. 


GEOGNOSY  277 

granite.  A  u  g  i  t  e-g  r  a  n  i  t  e — a  variety  in  which  augite  oc- 
curs with  black  mica.  Tourmaline  granite — agranitite 
with  disseminated  tourmaline.  Greisen — a  rare  granitic 
rock  from  which  the  felspar  has  disappeared,  found  in  some 
granite  districts,  especially  in  those  wherein  mineral-veins 
occur.  A  p  1  i  t  e — a  fine-grained  mixture  of  quartz  and  fel 
spar,  which  have  not  infrequently  intergrown  (micropeg- 
matite);  found  especially  in  veins  in  granite.  "Elvan  is 
a  Cornish  term  for  a  crystalline-granular  mixture  of  quartz 
and  ortboclase,  forming  veins  which  proceed  from  granite, 
or  occur  only  in  its  neighborhood,  and  are  evidently  asso- 
ciated with  it.109  Under  the  name  Granulite  M.  Michel- 
LeVy  includes  certain  fine-grained  granites  with  white  mica, 
'which  to  the  naked  eye  appear  to  be  composed  entirely  of 
felspar  and  quartz,  or  of  felspar  alone,  though  both  mica 
and  quartz  appear  in  abundance  when  the  rocks  are  micro- 
scopically examined.  He  includes  in  this  category  most  of 
the  rocks  of  the  Alps  described  as  "protogine." 

Surrounding  large  masses  of  granite  there  are  usually 
numerous  veins,  which  consist  of  granite,  quartz- porphyry, 
felsite,  or  sometimes  even  spherulitic  material  (Mull).  There 
can  be  no  doubt  that  these  finer-grained  protrusions  really 
proceed  from  the  crystalline  granite  mass.  Lessen  has  shown 
that  the  Bode  vein  in  the  Harz  has  a  granitoid  centre,  with 
compact  porphyry  sides,  in  which  he  found  with  the  micro- 
scope a  true  glassy  base.170  Sometimes  the  rocks  associated 
in  this  wav  with  granite  differ  in  composition  from  the  main 
granite.  'Tourmaline  is  one  of  the  characteristic  minerals 
of  granite-veins,  though  less  observable  in  the  main  body  of 
the  rock;  with  quartz,  it  forms  Schorl-rock. 

Granite  weathers  chiefly  by  the  decay  of  its  felspars. 
These  are  converted  into  kaolin,  the  mica  becomes  yellow 
and  soft,  while  the  quartz  stands  out  scarcely  affected.  The 
granite  of  the  southwest  of  England  has  weathered  to  a 
depth  of  50  feet  and  upward,  so  that  it  can  be  dug  out  with 
a  spade,  and  is  largely  used  as  a  source  of  porcelain-clay.  <. 

Granite  occurs  (1)  as  an  eruptive  rock,  forming  huge 
bosses,  which  rise  through  other  formations  both  stratified 
and  unstratified,  and  sending  out  veins  into  the  surrounding 
and  overlying  rocks,  which  usually  show  evidence  of  much 
alteration  as  they  approach  the  granite ;  (2)  connected  with 

169  J.  A.  Phillips,  Q.  J.  Geol.  Soc.  xxxi.  p.  334.  Michel-IxSvy,  Bull.  Soc. 
Gteol.  France,  iii.  3d  ser.  p.  201. 

110  Zeitsch.  Deutsch.  Geol.  Gea.  xxvi.  (1874),  p.  866. 


278  TEXT-BOOK    OF   GEOLOGY 

true  volcanic  rocks  (as  in  the  Tertiary  granophyres  of  Mull 
and  Skye),  and  forming,  perhaps,  the  lower  portions  of 
masses  which  flowed  out  at  the  surface  as  lavas.  Granite 
is  thus  a  decidedly  plutonic  rock;  that  is,  it  has  consolidated 
at  some  depth  beneath  the  surface,  and  in  this  respect  differs 
from  the  superficial  volcanic  rocks,  such  as  lavas,  which 
have  flowed  out  above  ground  from  volcanic  orifices. 

Quartz- Porphyry  (Microgranite,  Eurite)."1 — A  fine-grained 
microgranitic  ground-mass,  composed  mainly  of  felspar  and 
quartz,  through  which  are  usually  scattered  conspicuous 
porphyritic  crystals  of  one  or  other  or  both  of  the  same 
minerals. 

To  the  naked  eye  the  ground-mass  varies  from  an  ex- 
ceedingly compact  texture  to  one  where  abundant  minute 
c^stals  can  be  detected.  Of  the  porphyritic  constituents 
the  quartz  occasionally  occurs  in  bi-pyramidal  crystals;  the 
felspar  is  usually  orthoclase,  while  black  mica  occasionally 
appears.  Under  the  microscope  the  structure  of  the  rock  is 
found  to  be  microgranitic,  with  frequently  a  micropegma- 
titic  arrangement  of  the  quartz  and  felspar  (granophyre). 

The  flesh-red  quartz- porphyry  of  Dobritz,  near  Meissen, 
in  Saxony,  was  found  by  Kentzsch  to  have  the  following 
chemical  composition:  Silica,  76-92:  alumina,  12-89;  pot- 
ash, 4-27;  soda,  0-68;  lime,  0-68;  magnesia,  0*68;  oxide  of 
iron,  1-15;  water,  1-97;  total,  99-54— specific  gravity,  2-49. 

The  colors  of  the  rock  depend  chiefly  upon  those  of 
the  felspar — pale  flesh-red,  reddish-brown,  purple,  yellow, 
bluish  or  slate-gray,  passing  into  white,  being  in  different 
places  characteristic.  It  will  be  observed  in  this,  as  in  other 
rocks  containing  much  felspar,  that  the  color,  besides  de- 
pending on  the  nue  of  that  mineral,  is  greatly  regulated  by 
the  nature  and  stage  of  decomposition.  A  rock,  weathering 
externally  with  a  pale  yellow  or  white  crust,  may  be  found 
to  be  dark  in  the  central  undecayed  portion.  When  the 
base  is  very  compact,  and  the  felspar-crystals  well-defined 
and  of  a  different  color  from  the  base,  the  rock,  as  it  takes 
a  good  polish,  may  be  used  with  effect  as  an  ornamental 
stone.  In  popular  language,  such  a  stone  is  classed  with 
the  "marbles,  under  the  name  of  "porphyry." 

The  Quartz-porphyries  occur  (1)  with  plutonic  rocks, 
as  eruptive  bosses  or  veins,  often  associated  with  granite, 
from  which,  indeed,  they  may  be  seen  to  proceed  directly; 

171  Zirkel,  "Microscop.  Petrog."  p.  71.  See  particularly  Rosenbusch,  "Mik. 
Phys."  ii.  p.  60. 


GEOGNOSY  279 

of  frequent  occurrence  also  as  veins  and  irregularly  intruded 
masses  among,  highly  convoluted  rocks,  especially  when 
these  have  been  more  or  less  metamorphosed;  (2)  in  the 
chimneys  of  old  volcanic  orifices,  forming  there  the  "neck" 
or  plug  by  which  a  vent  is  filled  up;  and  (3)  as  bosses  some- 
times of  large  size  which  have  been  protruded  in  connection 
with  volcanic  action.  Between  the  granophyres  which  are 
characterized  by  a  micropegmatitic  structure  and  the  fel- 
sites  or  ancient  rhyolites  there  is  a  close  relation.  Quartz- 
porphyries  are  abundant  in  Britain  among  formations  of 
Lower  Silurian,  Old  Red  Sandstone  and  Lower  Carbonifer- 
ous age.  In  the  Inner  Hebrides  they  occur  in  large  bosses 
or  domes  (granophyre)  rising  through  the  older  Tertiary 
basaltic  plateau. 

Many  of  the  rocks  called  "quartz-porphyry"  are  not 
microgranitic  but  have  the  "felsitic"  structure  arising  from 
the  devitrification  of  ancient  forms  of  rhyolite  (see  p.  280). 

Rhyolite172  (Liparite,  Quartz -trachyte) — a  rock  having  a 
compact  pale-gray,  yellowish,  greenish  or  reddish  ground- 
mass,  sometimes  with  glassy  patches  and  layers,  often 
showing  perfect  flow-structure,  not  infrequently  also  with 
spherulitic  and  perlitic  structures,  and  with  crystals  of 
orthoclase  (sanidine),  granules  of  quartz  and  minute  crystals 
of  black  mica,  augite,  more  rarely  hornblende.  Consider- 
able diversity  exists  in  the  texture  of  the  rock.  Frequently 
it  is  finely  cavernous,  the  cavities  being  lined  with  chal- 
cedony, quartz,  amethyst,  jasper,  etc.  Some  varieties  are 
coarse  and  granitoid  in  character.  Intermediate  varieties 
may  be  obtained  like  the  quartz- porphyries,  and  these  pass 
by  degrees  into  more  or  less  distinctly  vitreous  rocks. 
Throughout  these  gradations,  however,  which  doubtless 
represent  different  stages  in  the  crystallization  of  an  orig- 
inal molten  glass,  a  characteristic  ground-mass  can  be  seen 
under  the  microscope  having  a  glassy,  enamel-like,  porce- 
laneous,  microlitic  character,  with  characteristic  spherulitic 
and  fluxion  structures.  In  the  quartz,  glass-inclusiofis, 
having  a  dihexahedral  form,  may  often  be  detected;  but 
liquid  inclusions  are  absent.  An  analysis  by  Vom  Rath 
of  a  rhyolite  from  the  Euganean  Hills  'gave — silica,  76-03; 
'alumina,  13*32;  soda,  5-29;  potash,  3*83;  protoxide  of  iron, 
1-74;  magnesia,  0-30;  lime,  0-85;  loss,  0-32;  total,  101-68— 
specific  gravity,  2 '553. 

1W  On  rhyolite  see  Richthofen,  Jahrb.  K.  K.  Geol.  Reichsanst.  xi.  156. 
Zirkel,  "Micro.  Petrog."  p.  163.  King,  "Explor.  40th  Parallel,"  vol.  i.  p.  606. 


280  TEXT-BOOK   OF   GEOLOGY 

The  perlitic  structure  is  so  characteristic  of  this  rock 
that  the  varieties  which  specially  exhibit  it  were  formerly 
regarded  as  a  distinct  rock-species  under  the  name  of  Perlite 
or  Pearlstone.  As  the  name  indicates,  the  structure  presents 
enamel-like  or  vitreous  globules  which,  occasionally  assum- 
ing polygonal  forms  by  mutual  pressure,  sometimes  consti- 
tute the  entire  rock,  their  outer  portions  shading  off  into 
each  other,  so  as  to  form  a  compact  mass;  in  other  cases, 
separated  by  and  cemented  in  a  compact  glass  or  enamel. 
They  consist  of  successive  very  thin  shells,  which,  in  a 
transverse  section,  are  seen  as  coiled  or  spiral  rings,  usually 
full  of  the  same  kind  of  hair-like  crystallites  and  crystals  as 
in  the  more  glassy  parts  of  the  rhyolite  (Fig.  9).  As  these 
bodies  both  singly  and  in  fluxion-streams  traverse  the  glob- 
ules, the  latter  may  be  regarded  as  a  structure  developed  by 
contraction  in  the  rock,  during  its  consolidation,  analogous 
to  the  concentric  spheroidal  structure  seen  in  weathered 
basalt  (Fig.  94).  Among  these  concentrically  laminated 
globules  true  spherulites  occur,  distinguished  by  their  in- 
ternal radiating  fibrous  structure  (Figs.  7,  17). 

Rhyolite  is  an  acid  rock  of  volcanic  origin.  It  forms 
enormous  masses  in  the  heart  of  extinct  volcanic  districts  in 
Europe  (Hungary,  Euganean  Hills,  Iceland,  Lipari),  and 
in  North  America  (Wyoming,  Utah,  Idaho,  Oregon,  Cali- 
fornia). 

N  e  v  a  d  i  t  e — a  variety  of  rhyolite  named  by  Richthofen 
from  its  development  in  Nevada,  and  characterized  by  its 
resemblance  to  granite,  owing  to  the  abundance  of  its  por- 
phyritic  crystals,  and  the  relatively  small  amount  of  ground- 
mass  in  which  they  are  imbedded.  The  granitoid  aspect  is 
external  only,  as  the  ground-mass  is  distinct,  and  varies 
from  a  holocrystalline  character  to  one  with  abundant  glass, 
and  the  texture  ranges  from  dense  to  porous. ITS 

F  e  1  s  i  t  e  (Felstone). — Under  this  name  a  large  series  of 
rocks  has  been  grouped  which  appear  for  the  most  part  to 
have  been  originally  vitreous  lavas  like  the  rhyolites,  but 
which  have  undergone  complete  devitrification,  though  fre- 
quently retaining  the  perlitic,  spherulitic,  and  flow-struc- 


113  Hague  and  Iddings,  Amer.  Journ.  Sci.  xxvii.  (1884),  p.  461.  These  au- 
thors distinguish  between  Nevadite  and  Liparite,  the  latter  being  characterized 
by  the  small  number  of  porphyritic  crystals  imbedded  in  a  relatively  large 
amount  of  ground-mass,  which,  as  in  Nevadite,  may  be  holocrystalline  or 
glassy.  They  also  distinguish  Lithoidal  Rhyolite  and  Hyaline  RJiyolite  as 
additional  varieties. 


GEOGNOSY  281 

tures.  They  vary  in  color  from  nearly  white  through  shades 
of  gray,  blue  and  red  or  brown  to  nearly  black,  often  weath- 
ering with  a  white  crust.  They  are  close-grained  in  texture, 
often  breaking  with  a  sub-conchoidal  fracture  and  showing 
translucent  edges.  Porphyritic  felspars  (both  orthoclase  and 
plagioclase)  and  blebs  of  quartz  are  of  frequent  occurrence. 
The  flow-structure  is  occasionally  strongly  marked  by  bands 
of  different  color  and  texture,  sometimes  curiously  bent  and 
curled  over,  indicating  the  direction  of  movement  of  the 
still  unconsolidated  rock.  The  spherulitic  structure  also 
may  be  found  so  strongly  marked  that  the  individual  spher- 
ules measure  an  inch  or  more  in  diameter,  so  that  the  rock 
seems  composed  of  an  aggregate  of  balls,  and  was  formerly 
mistaken  for  a  conglomerate  (Pyromeri.de).1'1*  Under  the 
microscope  many  of  the  typical  structures  of  rhyolite  can 
be  detected  in  felsites.  The  ground-mass  of  these  rocks  has 
given  rise  to  much  discussion,  but  it  is  now  generally  recog- 
nized as  a  more  or  less  altered  condition  of  the  devitrifi- 
cation of  an  original  vitreous  mass  (p.  207).  Secondary 
changes  have  in  large  measure  destroyed  the  original  micro- 
litic  structure,  but  traces  of  it  can  often  be  found,  while  the 
spherulitic  and  perlitic  forms  frequently  remain  almost  as 
fresh  as  in  a  recent  rock.  Felsites  with  a  large  propor- 
tion of  alkalies,  especially  soda,  have  been  called  Kerato- 
phyres.1''* 

Felsites  have  been  found  abundantly  as  interbedded 
lavas  with  tuffs  and  agglomerates  associated  with  Silurian 
and  older  rocks  in  Wales  and  Shropshire.17"  Soda-felsites 
or  keratophyres  have  been  found  to  play  a  considerable  part 
among  the  materials  erupted  by  the  Lower  Silurian  vol- 
canoes of  the  southeast  of  Ireland.177 

The  vitreous  acid  rocks  form  an  interesting  group  in  which 
we  may  still  detect  what  was  probably  the  original  condition 


114  On  nodular  felsites  see  G.  Cole,  Quart.  Journ.  Geol.  Soc.  xli.  (18Sb), 
p.  162;  xlii.  p.  183;  Miss  Raisin,  op.  cit.  xlv.  (1889),  p.   247.     Harker  "Bala 
Volcanic  Bocks,"  1889,  p.  28. 

115  Gumbel,  "Palaeolit.  Eruptivgest.  Fichtelgebirg."  (1874),  p.  43.     Rosen- 
busch,  "Mikrosop.  Physiog."  ii.  434. 

116  Mr.  Allport  described  some  ancient  forms  of  perlitic  structure  fronl  Shrop- 
shire, in  what  were  probably  once  ordinary  rhyolites,  Q.  J.  Geol.  Soc.  xxxiii. 
p.  449 ;  and  Mr.  Rutley  showed  the  presence  of  the  same  structure  among  the 
Lower  Silurian  lavas  of  North  Wales.     Op.  cit.  xxxv.  p.  508. 

111  F.  H.  Hatch,  Mem.  Geol.  Surv.  Ireland,  Explanation  of  Sheet  130;  Geol. 
Mag.  1889,  p.  70. 


282  TEXT-BOOK    OF   GEOLOGY 

of  at  least  the  rhyolites  and  felsites.  Every  gradation  can 
be  traced  from  a  perfect  glass  into  a  thoroughly  devitrified 
and  even  crystalline  rock.  As  already  remarked,  the  origi- 
nal vitreous  condition  of  rhyolite  can  still  be  seen  even  with 
the  naked  eye  in  the  clots  and  streaks  of  glass  that  occa- 
sionally run 'through  it  in  the  direction  of  its  flow-structure. 
Various  names  have  been  given  to  the  glassy  rocks,  of  which 
the  chief  are  obsidian,  pitchstone  and  pumice.  These,  how- 
ever, are  not  to  be  regarded  as  distinct  rock-species  but 
rather  as  the  glassy  condition  of  different  lavas. 

Obsidian  (rhyolite-glass) — the  most  perfect  form  of 
volcanic  glass,  externally  resembling  bottle  glass,  having  a 
perfect  conchoidal  fracture,  and  breaking  into  sharp  splinters, 
transparent  at  the  edges.  Its  colors  are  black,  brown,  or 
grayish-green,  rarely  yellow,  blue,  or  red,  but  not  infre- 
quently streaked  or  banded  with  paler  and  darker  hues.  A 
thin  slice  of  obsidian  prepared  for  the  microscope  is  found 
to  be  very  pale  yellow,  brown,  gray,  or  nearly  colorless,  and 
on  being  magnified  shows  that  the  usual  dark  colors  are  al- 
most always  produced  by  the  presence  of  minute  opaque  crys- 
tallites, which  present  themselves  as  black  opaque  trichytes, 
sometimes  beautifully  arranged  in  eddy-like  lines  showing 
the  original  fluid  movement  of  the  rock  (Fig.  19);  also  as 
rod-like  transparent  microlites.  They  occasionally  so  in- 
crease in  abundance  as  to  make  the  rock  lose  the  aspect  of 
a  glass  and  assume  that  of  a  dull  flint-like  or  enamel-like 
stone.  This  devitrification  can  only  be  properly  studied 
with  the  microscope.  Again,  dull  gray  enamel-like  spheru- 
lites  appear  in  some  parts  of  the  rock  in  great  abundance, 
drawn  out  intd  layers  so  as  to  give  the  rock  a  fissile  struc- 
ture, while  steam-  or  gas-cavities  likewise  occur,  sometimes 
so  large  and  abundant  as  to  impart  a  cellular  aspect.  The 
occurrence  of  abundant  sanidine  crystals  gives  rise  to  Por- 
phyritic  Obsidian.  Many  obsidians,  from  the  increase  in  the 
number  of  their  steam-vesicles,  pass  into  pumice.  Now  and 
then,  the  steam-pores  are  found  in  enormous  numbers,  of  ex- 
tremely minute  size,  as  in  an  obsidian  from  Iceland,  a  plane 
of  which,  about  one  square  millimetre  in  size,  has  been  esti- 
mated to  include  800,000  pores.  The  average  chemical  com- 
position of  obsidian  is — silica,  71-0;  alumina,  13-8;  potash, 
4'0;  soda,  5-2;  lime,  !•!;  magnesia,  0'6;  oxides  of  iron  and 
manganese,  3-7;  loss,  0-6  (little  or  no  water).  Mean  specific 
gravity,  2*40.  Obsidian  occurs  as  a  product  of  the  volca- 
noes of  late  geological  periods.  It  is  found  in  Lipari,  Ice- 
land, and  Teneriffe;  in  North  America,  it  has  been  erupted 


GEOGNOSY  283 

from  many  points  among  the  Western  Territories;178  it  is 
met  with  also  in  New  Zealand. 

Pitchstone  is  a  name  given  to  the  less  perfectly 
glassy  acid  rocks,  which  are  distinguished  by  a  resinous 
or  pitch-like  lustre,  and  internally  by  a  more  advanced  de- 
velopment of  microlites  than  in  obsidian.  They  thus  rep- 
resent a  further  stage  of  devitrification.  These  rocks  are 
easily  frangible,  breaking  with  a  somewhat  splintery  frac- 
ture, translucent  on  thin  edges,  with  usually  a  black  or  dark 
green  color,  that  ranges  through  shades  of  green,  brown,  and 
yellow  to  nearly  white.  Examined  microscopically,  they 
are  found  to  consist  of  glass  in  which  are  diffused  hair-like, 
feathery  and  rod-shaped  microlites,  or  more  definitely 
formed  crystals  of  orthoclase,  plagioclase,  quartz,  horn- 
blende, augite,  magnetite,  etc.  The  pitchstone  of  Corrie- 
gills,  in  the  island  of  Arran,  presents  abundant  green, 
leathery,  and  dendritic  microlites  of  hornblende  (Fig.  14). l" 
Occasionally,  as  in  Arran,  pitchstone  assumes  a  spherulitic 
or  perlitic  structure.  Sometimes  it  becomes  porphyritic,  by 
the  development  of  abundant  sanidine  crystals  (Isle  of  Eigg). 

Pitchstone  is  found  as  (1)  intrusive  dikes,  veins,  or 
bosses,  probably  in  close  connection  with  former  volcanic 
activity,  as  in  the  case  of  the  dikes,  which  in  Arran  traverse 
Lower  Carboniferous  rocks,  but  are  probably  of  Miocene 
age,  and  those  which  in  Meissen  send  veins  through  and 
overspread  the  younger  Palaeozoic  felsite-porphyries;  (2) 
sheets  which  have  flowed  at  the  surface,  as  in  the  remark- 
able mass  forming  the  Scuir  of  Eigg,  which  has  filled  up  a 
river-channel  of  older  Tertiary  age.180  * 

Pumice  (Ponce,  Bimstein) — a  general  term  for  the 
loose,  spongy,  cellular,  filamentous  or  froth-like  parts  of 
lavas.  So  distinctive  is  this  structure,  that  the  term  pumi- 
ceous  has  come  into  general  use  to  describe  it.  There  can  be 
no  doubt  that  this  froth-like  rock  owes  its  peculiarity  to  the 
abundant  escape  of  steam  or  gas  through  its  mass  while  still 
in  a  state  of  fusion.  The  most  perfect  forms  of  pumice  are 
found  among  the  acid  lavas,  but  this  type  of  rock  may  be 
met  with  in  the  other  groups.  Microscopic  examination  of 
a  rhyolitic  pumice  reveals  a  glass  crowded  with  enormous 

118  For  an  account  of  the  obsidian  of  the  Yellowstone  Park  see  J.  P.  Iddmgs, 
tth  Kept.  U.  S.  Geol.  Surv.  (1885-86),  p.  255;  consult  also  Zirkel,  "Microscop. 
Petrog." 

119  See  F.  A.  Gooch,  Min.  Mittheil.  1876,  p.  185.     Allport,  Geol.  Mag.  1881, 
p.  438. 

110  Quart.  Journ.  Geol.  Soc.  (1871),  p.  303. 


284  TEXT-BOOK    OF    GEOLOGY 

numbers  of  minute  gas-  or  vapor-cavities,  usually  drawn  out 
in  one  direction,  also  abundant  crystallites  like  those  of  ob- 
sidian. Owing  to  its  porous  nature,  pumice  possesses  great 
buoyancy  and  readily  floats  on  water,  drifting  on  the  ocean 
to  distances  of  many  hundreds  of  miles  from  land,  until  the 
cells  are  gradually  filled  with  water,  when  the  floating  masses 
sink  to  the  bottom.1"1  Abundant  rounded  blocks  of  purnice 
were  dredged  up  by  the  "Challenger"  from  the  floor  of  the 
Atlantic  and  Pacific  Oceans. 

ii.   Intermediate  Series 

In  this  series,  the  average  percentage  of  silica  is  consid- 
erably less  than  in  the  acid  series  (56-66  per  cent).  Free 
quartz  is  not  found  as  a  marked  constituent,  although  occa- 
sionally it  occurs  in  some  quantity,  as  microscopic  examina- 
tion has  shown  in  the  case  even  of  some  rocks  where  the 
mineral  was  formerly  believed  to  be  absent.  A  range  of 
structure  is  displayed  similar  to  that  in  the  acid  series.  The 
thoroughly  crystalline  varieties  are  typified  by  syenite  (and 
diorite),  representing  the  granites  of  the  acid  rocks,  those 


among  '  the  trachytes  and  andesites  by  dark  glasses  of  the 
obsidian  and  pitchstone  types. 

Syenite.  —  This  name,  formerly  given  in  England  to  a 
granite  with  hornblende  replacing  mica,  is  now  restricted  to 
a  rock  consisting  essentially  of  a  holocrystalline  mixture  of 
orthoclase  and  hornblende,  to  which  -plagioclase,  biotite, 
augite,  magnetite,  or  quartz  may  be  added.  As  already 
mentioned,  the  word,  first  used  by  Pliny  in  reference  to  the 
rock  of  Syene,  was  introduced  by  Werner  as  a  scientific 
designation.  It  was  applied  by  him  to  the  rock  of  the 
Plauenscher-Grund,  Dresden;  he  afterward,  however,  made 
that  rock  a  greenstone.  The  base  of  all  syenites,  like  that 
of  granites,  is  thoroughly  crystalline,  without  an  amorphous 
ground-mass.  The  typical  syenite  of  the  Plauenscher- 
Grund,  formerly  described  as  a  coarse-grained  mixture  of 
flesh-colored  orthoclase  and  black  hornblende,  containing 
no  quartz,  and  with  no  indication  of  plagioclase,  was  re- 
garded as  a  normal  orthoclase-  hornblende  rock.  Micro- 

181  On  porosity,  hydration,  and  flotation  of  pumice,  see  Bischof,  "Chem.  und 
Pbys.  Geol."  suppl.  (1871),  p.  177. 


GEOGNOSY  280 

scopical  research  has,  however,  shown  that  well-striated 
tri clinic  felspars,  as  well  as  quartz,  occur  in  it.  Its  com- 
position is:  silica,  59 '3;  alumina,  16'85;  protoxide  of  iron, 
7-01;  lime,  4-43;  magnesia,  2-61;  potash,  6-57;  soda,  2;44; 
water,  etc.,  1'29;  total,  101  -03.  Average  specific  gravity, 
2-75  to  2-90. 

Syenite  is  of  much  less  frequent  occurrence  than  granite. 
While  always  thoroughly  granitic  in  structure,  it  varies  in 
texture  from  coarse  granular,  where  the  individual  minerals 
can  readily  be  distinguished  by  the  naked  eye,  to  compact. 
Among  its  accessory  minerals  of  common  occurrence  may 
be  mentioned  titanite  (sphene),  quartz,  apatite,  epidote, 
orthite,  magnetite,  pyrite,  zircon.  The  predominance  of 
one  or  more  of  the  ingredients  has  given  rise  to  the  sepa- 
ration of  a  few  varieties  under  distinctive  names.  In  the 
typical  syenite,  the  dark  silicate  is  almost  wholly  horn- 
blende; sometimes  there  are  to  be  found  traces  of  augite 
within  the  hornblende,  indicating  that  the  former  mineral 
was  the  original  constituent  and  has  been  changed  by  para- 
morphism.  Where  the  ferro-magnesian  silicate  is  mainly 
augite,  as  in  the  well-known  rock  of  Monzoni,  the  rock  is 
termed  Augite-syenite  or  Monzonite;  where  brown 
mica  predominates  it  gives  rise  to  Mica-syneite  or  Mi- 
nette. 

Elaeolite-syenite  (Nepheline-syenite)  is  a  granitoid 
rock,  characterized  by  the  association  of  the  variety  of 
nepheline  known  as  elaeolite  with  orthoclase,  and  with 
minor  proportions  of  plagioclase,  microcline,  hornblende, 
augite,  biotite,  sodalite,  zircon,  and  sphene.  It  is  distin- 
guished by  the  rare  minerals,  upward  of  fifty  in  number, 
which  it  contains,  and  in  which  some  of  the  rarer  elements 
are  combined,  such  as  thorium,  yttrium,  cerium,  lanthanum, 
tantalum,  niobium,  zirconium,  etc.  It  is  typically  devel- 
oped in  Southern  Norway  (Brevig,  Laurvig).  Where  zir- 
con enters  as  an  abundant  constituent  the  rock  is  known  as 
Zircon-syenite.  Foyaite  is  the  name  given  t<s>  a 
hornblendic  variety  found  at  Mount  Foya,  Portugal;  Mias- 
cite  is  a  variety  with  abundant  mica,  found  at  Miask; 
D  i  troi  te,  containing  eodalite,  spinel,  etc.,  occurs  at  Ditro 
in  Transylvania. 

Orthoclase-Porphyry  (Micro-syenite,  Quartzless-porphyry,  Or- 
thophyre)  stands  to  the  syenites  in  the  same  relation  that 
quartz-porphyry  or  micro-granite  does  to  the  granites.  It 
is  composed  of  a  compact  micro-granitic  ground-mass,  with 
little  or  no  free  quartz,  but  through  which  are  usually  scat- 


286  TEXT-BOOK   OF   GEOLOGY 

tered  numerous  crystals  of  orthoclase,  sometimes  also  a  tri- 
clinic  felspar,  black  hornblende  and  glancing  scales  of  dark 
biotite.  It  contains  from  55  to  65  per  cent  of  silica,  thus 
differing  from  quartz-porphyry  and  felsite  in  its  smaller  pro- 
portion of  this  acid.  It  is  also  rather  more  easily  scratched 
with  the  knife,  but  except  by  chemical  or  microscopical  ana- 
lysis, it  is  often  impossible  to  draw  a  distinction  between  this 
rock  and  its  equivalents  in  the  acid  series. 

Orthoclase-porphyry  occurs  in  veins,  dikes,  and  intru- 
sive sheets.  Probably  many  so-called  "felstones,"  whether 
occurring  as  lavas  or  as  intrusive  masses  among  the  older 
Palaeozoic  formations,  are  really  orthoclase-porphyries. 
Some  highly  micaceous  varieties  have  been  called  Mica- 
trap — a  vague  term  under  which  have  also  been  in- 
cluded Minettes,  Micaceous  Quartz-porphyries,  etc.  The 
name  Lamprophyre,  originally  given  by  Giimbel  to 
some  rnica-traps  from  the  Fichtelgebirge,  has  been  pro- 
posed by  Rosenbusch  as  a  general  term  for  the  Mica-traps, 
divisible  into  two  groups — the  Orthoclastic,  or  syenitic, 
where  the  felspar  is  orthoclase  (Minettes),  and  the  Plagio- 
clastic  or  dioritic,  where  the  felspar  is  a  plagioclase  variety 
(Kersantites).182  The  lamprophyres  occur  abundantly  as 
dikes  or  veins  of  a  fine-grained  texture,  and  dull  reddish 
to  brownish  color,  among  the  older  Palaeozoic  rocks  of 
Britain.183 

The  orthoclase-porphyry  of  Pieve  in  the  Vicentin  was 
found  by  Von  Lasaulx  to  have  the  following  composition: — 
silica,  61 '07;  alumina,  18 '56;  peroxides  of  iron  and  man- 
ganese, 2-60;  potash,  6-83;  soda,  3-18;  lime,  2-86;  mag- 
nesia, 1'18;  carbonic  acid,  1-36;  loss,  2p13 — specific  gravity, 
2-59.m 

Dioritc.1" — Under  this  name  is  comprehended  a  group  of 


IM  The  typical  locality  for  these  rocks  is  Kersanton  in  Brittany,  where  they 
are  dark-green  and  remarkably  durable.  A  singular  vein  of  kersantite,  3  to  Q\ 
feet  broad,  has  been  traced  for  nearly  five  miles  in  the  Harz.  Lessen,  Zeitsch. 
Deutsch.  Geol.  Ges.  xxxii.  (1880)  p.  445.  Jahrb.  Preuss.  Geol.  Landesanst. 
1880.  A.  von  Groddeck,  op.  cit.  1882.  M.  Koch,  op.  cit.  1886.  Barrois, 
Assoc.  Francaise  (1880),  p.  661 ;  Ann.  Soc.  Geol.  Nord,  xiv.  (1886),  p.  31. 

188  For  an  account  of  the  Lamprophyres  of  the  classical  district  of  the  Plauen- 
scher-Grund,  see  B.  Doss,  Tschermak's  Mineral  Mittheil.  xi.  (1889). 

184  Zeitsch.  Deutsch.  GeoL  Ges.  xxv.  p.  320. 

185  On  diorite,  its  structure  and  geological  relations,  consult  the  memoir  on 
Belgian  plutonic  rocks  by  De  la  Vallee  Poussin  and   A.  Renard,  Mem.  Acad. 
Royale  Belg.  1876;  Behrens,  Neues  Jalirb.  Min.  1871,  p.  460;  Zirkel,  "Micro- 
Bcopical  Petrog."  p.  83.  J.  A.  Phillips,  Q.  J.  Geol.  Soc.  xxxii.  p.  165,  and  xxxiv. 
p.  471— two  valuable  papera  in  which  the  constitution  of  some  of  the  "green- 


GEOGNOSY  287 

rocks,  which,  possessing  a  granitic  structure,  differ  from  the 
granites  in  their  much  smaller  percentage  of  silica,  and  from 
the  syenites  in  containing  plagioclase  instead  of  orthoclase 
as  their  chief  constituent.  They  are  sometimes  divided  into 
two  sections,  the  quartz-diorites  and  the  normal  diorites. 
Many  of  these  rocks  were  formerly  included  in  the  general 
division  of  "Greenstones. " 

Q  u  a  r  t  z-d  i  o  r  i  t  e — a  holocrystalline  mixture  of  plagio- 
clase (oligoclase,  less  frequently  labradorite)  and  quartz  with 
some  hornblende,  augite,  or  mica.  It  outwardly  resembles 

fray  granite,  and,  indeed,  includes  many  so-called  granites, 
ts  silica  ranges  up  to  67  per  cent.  In  normal  Diorite, 
quartz  is  almost  entirely  absent;  hornblende  and  black  mica 
occur  together  in  some  varieties,  while  pyroxene  character- 
izes others.  Under  the  microscope  a  thoroughly  crystalline 
structure  is  seen,  and  among  the  pyroxene-diorites  the  fel- 
spar and  pyroxene  are  sometimes  intergrown  in  ophitic  ag- 
gregates. The  average  chemical  composition  of  quartzless 
diorite  is:  silica,  54;  alumina,  16-18;  potash,  1-5-2-5;  soda, 
2-3;  lime,  6-7 '5;  magnesia,  6-0;  oxides  of  iron  and  man- 
ganese, 10-14;  mean  specific  gravity,  about  2'95. 

Among  the  varieties  of  diorite,  the  following  may  be 
mentioned.  C  o  r  s  i  t  e  (from  Corsica) — a  granitoid  mixture 
of  grayish-white  plagioclase,  blackish-green  hornblende,  and 
some  quartz,  which  have  grouped  themselves  into  globular 
aggregations  with  an  internal  radial  and  concentric  structure 
(Orbicular  diorite,  Kugeldiorit,  Napoleonite 
— Fig.  8).  T  o  n  a  1  i  t  e  (from  Monte  Tonale,  Tyrol) — a  vari- 
ety containing  quartz,  hornblende,  and  biotite  in  strongly 
contrasted  colors.  E  p  i  d  i  o  r  i  t  e — a  name  given  to  ancient 
rocks  which  have  originally  been  pyroxenic  eruptive  masses, 
but,  by  metamorphism,  have  acquired  a  crystalline  re- 
arrangement of  their  constituents,  the  pyroxene  being 
changed  into  hornblende,  often  fibrous  or  actinolitic,  the 
felspar  becoming  granular,  and  the  whole  rock  having  ac- 
quired a  more  or  less  distinct  schistose  structure.  The  dark 
intrusive  sheets  associated  with  the  crystalline  schists  of  the 
Scottish  Highlands  and  the  north  of  Ireland  are  largely 
epi  diorites.  Some  of  these  rocks  are  quartziferous,  but 
many  of  them  belong  to  the  basic  series  (see  p.  1052). 
As  the  granites  pass  into  fine-grained  quartz-porphyries, 


stones"  of  the  older  geologists  is  clearly  worked  out.  Many  of  these  ancient 
rocks  are  there  shown  to  be  forms  of  doleritic  lava  and  the  change  of  their 
original  augite  into  hornblende  is  traced. 


288  TEXT-BOOK    OF   GEOLOGY 

and  the  syenites  into  compact  orthoclase-porphyries,  so  the 
diorites  have  their  close-textured  varieties,  which  are  com- 
prised under  the  general  term  Aphanite,  divisible  into 
Quartz-aphanite  and  Normal-aphanite.  The  general  charac- 
teristic of  these  rocks  is  that  the  constituent  minerals  be- 
come so  minute  as  to  disappear  from  the  naked  eye.  They 
are  dark  heavy  close-grained  masses.  They  merge  into  the 
basic  diabases  (p.  170). 

Trachyte186 — a  term  originally  applied  to  modern  volcanic 
rocks  possessing  a  characteristic  roughness  (17^1/9)  under  the 
finger,  is  now  restricted  to  a  compact,  usually  pale,  porphjr- 
ritic,  frequently  cellular,  rock,  consisting  essentially  of  sani- 
dine,  with  more  or  less  triclinic  felspar,  augite,  hornblende, 
and  biotite,  sometimes  with  apatite,  and  tridymite.  It  is  dis- 
tinguished from  rhyolite,  or  quartz-trachyte,  by  the  absence 
of  free  quartz,  and  by  the  smaller  proportion  of  vitreous  or 
microlitic  (rnicro-felsitic)  ground-mass.  The  sanidine  crys- 
tals present  abundant  steam-pores  and  glass-inclusions,  as 
well  as  hornblende-microlites  and  magnetite.  In  some 
varieties,  the  ground-mass  appears  to  be  entirely  com- 
posed of  microlites;  in  others,  minor  degrees  of  devitrifi- 
cation can  be  traced,  until  the  ground -mass  passes  into  a 
glass  (trachyte-glass,  obsidian).  The  trachytes  of  Hungary 
have  been  grouped  as  Augite-trachyte,  Amphibole-trachyte 
and  Biotite-trachyte.  Average  composition  of  Trachyte — 
silica,  60 '0-64 -0;  alumina,  17*0;  protoxide  and  peroxide  of 
iron,  6-0-8-0;  magnesia,  1/0;  lime,  3 -5;  soda,  4-0;  potash, 
2-0-2-5.  Average  specific  gravity,  2-65. 

Trachyte  is  an  abundantly  diffused  lava  of  Tertiary  and 
Post-tertiary  date.  It  occurs  in  most  of  the  volcanic  dis- 
tricts of  Europe  (Siebengebirge,  Nassau,  Transylvania,  Bay 
of  Naples,  Euganean  Hills);  in  the  Western  Territories  of 
the  United  States;187  in  New  Zealand.  It  also  occurs  among 
the  Carboniferous  lavas  of  Scotland. 


188  On  trachyte,  see  Zirkel,  "Micro.  Petrog."  p.  143.  King  in  vol.  i.  of 
"Explor.  40th  Parallel,"  p.  578.  On  the  relative  age  and  classification  of  Hun- 
garian trachytes,  Szabo,  Zeitsch.  Deutsch.  Geol.  Ges.  xxix.  p.  635,  and  "Compte 
rend.  Congres  Internationale  de  Geologic"  (1878),  Paris,  1880.  For  the  Scottish 
Carboniferous  trachytes  see  Presidential  Address  to  the  Geological  Society  1892, 
and  P.  H.  Hatch,  Trans.  Roy.  Soc.  Edin.  1892. 

187  It  would  appear  that  much  of  what  has  been  regarded  as  trachyte  in 
Western  America  is  andesite,  consisting  essentially  of  plagioelase,  and  not  of 
sanidine.  The  normal  trachytes  are  now  described  as  hornblende-mica-audesites, 
and  the  augite-trachytes  are  hyperethene-augite-andesites,  most  of  the  rest  being 
dacites,  and  some  of  them  rhyolites.  Hague  and  Iddings,  Amer.  Journ.  8d. 
xxvii.  (1884),  p.  456. 


GEOGNOSY  289 

D  o  m  i  t  e  (so  named  from  the  Puy-de-D6me)  is  a  porous 
loosely  aggregated  trachyte,  having  a  microhtic  ground- 
mass,  through  which  are  dispersed  tridymite,  sanidine, 
much  plagioclase,  hornblende,  magnetite,  biotite,  and  spec- 
ular iron.  Soda-trachyte  (Pantellerite)  is  a  variety  rich 
in  oligoclase,  found  in  Pantelleria. 

Phonolite  (Nepheline-trachyte,  Clinkstone)188 — a  term  sug- 
gested by  the  metallic  ringing  sound  emitted  by  the  fresh 
compact  varieties  when  struck,  is  applied  to  a  compact,  gray 
or  brown,  quartzless  mixture  of  sanidine  and  nepheline, 
with  nosean,  hauyne,  leucite,  pyroxene,  hornblende,  or 
mica.  The  rock  is  rather  subject  to  decomposition,  hence 
its  fissures  and  cavities  are  frequently  filled  with  zeolites. 
An  average  specimen  gave  on  analysis — silica,  57-7;  alu- 
mina, 2O6;  potash,  6-0;  soda,  7gO;  lime,  1'5;  magnesia,  0'5; 
oxides  of  iron  and  manganese,  3'5;  loss  by  ignition,  3*2  per 
cent.  The  specific  gravity  may  be  taken  as  about  2-58. 
Phonolite  is  sometimes  found  splitting  into  thin  slabs 
which  can  be  used  for  roofing  purposes.  Occasionally  it 
assumes  a  porphyritic  texture  from  the  presence  of  large 
crystals  of  sanidine  or  of  hornblende.  When  the  rock  is 
partly  decomposed  and  takes  a  somewhat  porous  texture, 
it  resembles  normal  trachyte. 

It  is  a  thoroughly  volcanic  rock,  and  generally  of  Ter- 
tiary date.  It  occurs  sometimes  filling  the  pipes  or  volcanic 
orifices,  sometimes  as  sheets  which  have  been  poured  out  in 
the  form  of  lava-streams,  and  sometimes  in  dikes  and  veins, 
as  in  Bohemia  and  Auvergne.  Some  of  the  great  bosses 
or  eruptive  vents  connected  with  the  trachyte  lavas  of  the 
Carleton  Hills,  Haddingtonshire,  have  recently  been  deter- 
mined by  Dr.  Hatch  to  be  true  phonolites. 

With  the  pbonolites  may  be  classed  Leucite- 
trachyte,  or  Le  uci  te-p  h  o  noli  te,  where  the  fel- 
spathoid  is  leucite  instead  of  nepheline,  and  Nosean- 
trachyte  (Nosean-phonolite),  or  Hauyne-trachyte 
(Hauyne-phonolite),  with  nosean  or  hauyne  taking  £he 
place  of  the  felspar  of  ordinary  phonolite. 

Andesite — a  name  originally  given  by  Von  Buch  to  some 
lavas  found  in  the  Andes,  is  now  applied  to  a  large  series 
of  rocks  distinguished  from  the  trachytes  in  that  their  fel- 
spar is  plagioclase,  and  passing  by  the  addition  of  olivine 

188  Boricky,  "Petrograph.  Stud.  Phonolitgestein.  Bohmens." — Archiv.  Lan- 
desdurchforchung  Bohmen,  1874.  G.  P.  Fohr,  "Die  Phonolite  des  Hegau's," 
Verb.  Phys.  Med.  Ges.  Wiirzburg,  xviii.  (1883).  P.  H.  Hatch,  Trans.  Roy. 
Soc.  Edin.  1892. 

GEOLOGY— Vol.  XXIX— 13 


290  TEXT-BOOK   OF   GEOLOGY 

into  dolerite  and  basalt.  In  fresh  examples  they  are  dark 
gray,  or  even  black  rocks  with  a  compact  ground-mass, 
through  which  striated  felspar  prisms  may  generally  be  ob- 
served. They  often  assume  cellular  and  porphyritic  struc- 
tures. At  the  one  end  of  the  series  stand  rocks  containing 
free  silica  (Dacite),  while  at  the  other  are  basalt-like  masses 
of  much  more  basic  composition  (Aguite-andesite).  Under 
the  microscope  the  ground-mass  presents  more  or  less  of 
a  pale  brownish  glass  with  abundant  felspar  microlites. 

Dacite  (Quartz-andesite) — composed  mainly  of  plagio- 
clase,  quartz,  and  mica,  with  a  varying  amount  of  sanidine 
as  an  accessory  constituent,  and,  by  addition  of  hornblende 
and  pyroxene,  graduating  into  hornblende-andesite.  The 
ground-mass  has  a  felsitic,  sometimes  spherulitic,  glassy, 
or  finely  granular  base.  Composition:  silica,  69-36;  alu- 
mina, 16-23;  iron  oxides,  2-41;  lime,  3-17;  magnesia,  1*34; 
alkalies,  7 -08;  water,  0-45.  Mean  specific  gravity,  2 -60. 
This  rock  is  extensively  developed  in  the  Great  Basin  and 
other  tracts  of  western  North  America  among  Tertiary 
and  recent  volcanic  outbursts. 

Hornblende-andesite189  consists  of  a  triclinic  fel- 
spar (usually  oligoclase),  with  hornblende,  augite,  or  mica. 
The  ground-mass  resembles  that  of  trachyte,  presenting 
sometimes  remains  of  a  pale  glass.  The  porphyritic  min- 
erals frequently  show  evidence  of  having  been  much 
corroded  before  consolidation.  Composition:  silica,  61 '12; 
alumina,  11-61;  oxides  of  iron,  11  -64;  lime,  4-33;  magnesia, 
0-61;  potash,  3-52;  soda,  3*85;  ignition,  4-35.  Hornblende- 
andesite  is  a  volcanic  rock  of  Tertiary  and  post-Tertiary 
date  found  in  Hungary,  Transylvania,  Siebengebirge,  and 
in  some  of  the  Western  Territories  of  the  United  States. 
According  to  researches  by  Messrs.  Hague  and  Iddings, 
gradations  from  this  rock  into  basalt  and  hypersthene- 
andesite  can  be  traced  in  California,  Oregon,  and  Washing- 
ton. These  rocks,  therefore,  cannot  be  said  to  have  sharply 
denned  and  distinct  forms.190  Under  the  name  of  Horn- 
blende-mica-andesite  American  petrographers  have  described 
a  frequent  variety  of  rock  throughout  the  Great  Basin,  char- 
acterized by  the  vitreous  appearance  of  its  felspar,  its  rough 
porous  trachyte-like  ground-mass,  and  the  presence  of  mica 


189  See  Zirkel,  "Microscop.   Petrog."  p.  122.     King,  in  vol.  i.  of  "Explor. 
40th  Parallel,"  p.  562.     Hague  and  Iddings,  Amer.  Journ.  Sci.   xxvi.   (1883), 
p.  230. 

190  Amer.  Jouru.  Sci.  Sept.  1883,  p.  233. 


GEOGNOSY  291 

as  an  essential  constituent.  This  term  will  include  a  large 
proportion  of  the  rocks  hitherto  classed  as  trachytes,  but 
in  which  the  felspar  proves  to  be  plagioclase  and  not 
sanidine.191 

P  y  r  o  x  e  n  e-a  n  d  e  s  i  t  e — consisting  of  labradorite  or 
oligoclase,  with  augite  (less  frequently  a  rhombic  pyrox- 
ene) and  abundant  magnetite,  sometimes  with  hornblende 
or  mica,  forming  a  dark  heavy  basalt-like  compound,  with 
a  compact  sometimes  more  or  less  distinctly  vitreous  ground- 
mass.  Composition:  silica,  57*15;  alumina,  16'10;  protox- 
ide of  iron,  13-0;  lime,  5 '75;  magnesia,  2*21;  potash,  1*81; 
soda,  3'88.  Mean  specific  gravity,  2'75-2-85. 

It  was  formerly  supposed  that  the  pyroxene  of  the  ande- 
sites  was  always  augite.  But  rhombic  forms  of  the  mineral 
have  now  been  frequently  detected.  Under  the  name  of 
Hypersthene-andesite,  certain  Tertiary  or  recent  rocks, 
stretching  over  vast  areas  in  Western  America,  have  been 
described  as  associated  with  other  andesites  and  basalts. 
They  are  black  to  gray,  or  reddish-gray,  in  color,  and  vary 
in  texture  from  dense,  thoroughly  crystalline  forms,  to 
others  approaching  white  glassy  pumice,  the  base  under 
the  microscope  ranging  from  a  brown  glass  to  a  holocrvstal- 
line  structure.  The  magnesian  silicate  is  pyroxene,  cniefly 
in  the  orthorhombic  form  as  hypersthene,  but  partly  also 
augite.  An  analysis  of  the  pumiceous  form  of  the  rock 
gave  62  per  cent  of  silica,  while  the  percentage  of  the  same 
constituent  in  the  glass  of  the  base  was  found  to  rise  to 
69.94."" 

Pyroxene-andesite  occurs  in  dikes,  lava-streams,  plateaus, 
sheets,  and  neck-like  bosses  in  regions  of  extinct  and  active 
volcanoes,  as  in  the  Inner  Hebrides,  Antrim,  Transylvania, 
Hungary,  Santorin,  Iceland,  Teneriffe,  the  Western  Terri- 
tories of  North  America,  the  Andes,  New  Zealand,  etc. 
Many  of  the  rocks  of  these  regions  now  classed  under  this 
name  have  long  been  known  and  described  as  dolerites  and 
basalts.  Indeed,  there  is  the  closest  relation  between  them 
and  the  true  olivine-bearing  dolerites  and  basalts.  The 
latter  occur  among  the  Tertiary  volcanic  plateaus  of  Britain, 
interstratified  with  rocks  which,  not  containing  olivine,  have 
been  placed  among  the  andesites.  Neither  in  their  mode  of 
occurrence  nor  to  the  eye  in  hand  specimens  is  there  any 

191  Hague  and  Iddings,  Amer.  Journ.  Sci.  xxvii.  (1884),  p.  460. 
92  Whitman  Cross,  Bull.   U.  S.  Geol.  Survey,   1883,  No.   1.     Hague  and 
Iddinga,  Amer.  Journ.  Sci.  xxvi.  (1883),  p.  226;  xxvii.  (1884),  p.  457. 


292  TEXT-BOOK    OF   GEOLOGY 

good  distinction  to  be  drawn  between  them.  Under  the 
name  of  T  hoi ei te  some  interesting  augite-andesites  have 
been  described,  in  which  the  felspar  prisms  form  a  network 
filled  in  with  granular  augite  and  interstitial  matter  (inter- 
sertal  structure).  In  other  varieties  of  andesite  the  felspar- 
mesh  has  been  filled  with  large  crystalline  patches  of  augite, 
which  thus  incloses  the  felspar  (ophitic  structure). 

Tephrite  (Nepheline-andesite,  Leucite-andesite,  No- 
Bean-  or  Hauyne-andesite) — a  group  of  andesites,  in  which 
the  felspar  is  partly  replaced  by  one  of  the  felspathoids, 
nepheline,  leucite,  nosean,  or  hauyne. 

Porphyrit e — a  name  for  old  forms  of  andesite  which 
have  generally  undergone  considerable  alteration,  and  conse- 
quently appear  as  dull,  sometimes  earthy,  generally  reddish 
or  brownish  rocks.  When  fresh  they-  are  dark  grav  or 
black.  They  are  commonly  porphyritic,  and  show  abun- 
dant scattered  crystals  of  plagioclase,  less  commonly  of 
mica.  Their  texture  varies  from  coarse  crystalline  to  ex- 
ceedingly close-grained,  passing  occasionally  into  vitreous 
varieties  (Yetholm,  Cheviot  Hills).  Rocks  of  this  type  have 
been  abundantly  poured  forth  as  lavas  during  Palaeozoic 
time,  and  they  occur  as  interstratified  lava-beds,  eruptive 
sheets,  dikes,  veins,  and  irregular  bosses.  In  Scotland  they 
form  masses,  several  thousand  feet  thick,  erupted  in  the 
time  of  the  Lower  Old  Red  Sandstone,  and  others  of  wide 
extent  and  several  hundred  feet  in  depth  belonging  to  the 
Lower  Carboniferous  period.  .  In  Germany  porphyrites  ap- 
pear also  at  numerous  points  among  formations  of  later 
Palaeozoic  age. 

Propylite — a  name  given  by  Richthofen  to  certain 
Tertiary  volcanic  rocks  of  Hungary,  Transylvania,  and  the 
Western  Territories  of  the  United  States,  consisting  of  a 
triclinic  felspar  and  hornblende  in  a  fine-grained  non-vitre- 
ous ground-mass,  and  closely  related  to  the  Hornblende- 
andesites.  Their  distinguishing  feature  is  the  great  altera- 
tion which  they  have  undergone,  whereby  their  ferro-mag- 
nesian  constituents  have  been  converted  into  chlorite,  and 
their  felspars  into  epidote.  Some  quartziferous  propy likes 
have  been  described  by  Zirkel  from  Nevada,  wherein  the 
quartz  abounds  in  liquid  inclusions  containing  briskly-mov- 
ing bubbles,  and  sometimes  double  inclosures  with  an  inte- 
rior of  liquid  carbon-dioxide.193  A  specimen  from  Storm 


193  Zirkel's  "Microscopical  Petrography,"  p.   110.     King,  "Exploration  of 
40th  Parallel,"  vol.  i.  p.  54=5.     C.  E.  Button's  "High  Plateaus  oi  Utah"  (U.  S. 


GEOGNOSY  293 

Canon,  Fish  Creek  Mountains,  contained  silica,  60-58; 
alumina,  17-52;  ferric  oxide,  2-77;  ferrous  oxide,  2-53; 
manganese,  a  trace;  lime,  3 -78;  magnesia,  2-76;  soda, 
3-30;  potash,  4-46;  carbonic  acid,  a  trace.  Loss  by  igni- 
tion, 2-25;  specific  gravity,  2-6-2-7.  The  geologists  of  the 
Geological  Survey  of  the  United  States  believe  that  the 
rocks  included  under  the  term  "propylite"  in  the  western 
parts  of  America  represent  various  stages  of  the  decomposi- 
tion of  granular  diorite,  porphyritic  diorite,  diabase,  quartz- 
porphyry,  hornblende-andesite,  and  augite-andesite.194  The 
name  has  been  more  recently  applied  by  Rosenbusch  to 
rocks  which  have  undergone  alteration  by  solfataric  action. 

iii.  Basic  Series 

This  third  series  of  eruptive  rocks  is  distinguished  by  its 
low  silica  percentage,  and  the  relative  abundance  of  its  basic 
constituents.  A  similar  range  of  structure  can  be  traced  in 
it  as  in  the  other  two  series.  At  the  one  extreme  come 
rocks  with  a  holocrystalline  structure  like  the  gabbros. 
These  pass  into  others  of  a  hemi-crystalline  character,  where, 
amid  abundant  crystals,  crystallites,  and  microlites,  there 
are  still  traces  of  the  original  glass.  At  the  other  end  lie 
true  basic  volcanic  glasses,  which  externally  might  be  mis- 
taken for  the  pitchstones  and  obsidians  of  the  acid  rocks. 

Gabbro195  (Euphotide) — a  group  of  coarsely  crystalline 
rocks  composed  of  plagioclase  (labradorite)  or  anorthite, 
magnetite  or  titaniferous  iron,  and  some  lerro-magnesian 
mineral,  which  in  the  normal  gabbros  is  augite  or  diallage, 
but  may  be  a  rhombic  pyroxene,  hornblende,  olivine,  or 
mica.  These  minerals  occur  in  allotriomorphic  forms,  as 
in  granite;  but  they  sometimes  assume  ophitic  relations 
which  lead  into  the  rock  termed  dolerite.  The  felspar  has 
often  lost  its  vitreous  lustre  and  passed  into  the  dull  opaque 
condition  known  as  saussurite.  The  augite  is  usually  in  the 
form  of  diallage,  distinguished  by  its  schiller-spar  lustre.' 

Geographical  and  Geological  Survey  of  the  Rocky  Mountains),  chaps,  iii.  and 
iv.  Hague  and  Iddings,  Amer.  Journ.  Sci.  1883. 

194  G.  F.  Becker  on  the  Comstock  Lode.     Reports  of  U.  S.  Geological  Survey 
1880-81,  aud  his  full  memoir  in  vol.  iii.  of  the  Monographs  of  U.  S.  Geol.  Sur- 
vey (1882).     Hague  and  Iddings,  Amer.  Journ.  Sci.  xxvii.  (1884),  p.  454. 

195  On  Gabbro  see  Lossen,  Z.  Deutoch.  GeoL  Ges.  xix.  p.  651.     Lang,  op. 
cit.  xxxi.  p.  484.     Zirkel  on  Gabbros  of  Scotland,  op.  cit.  xxiii.  1871.     Judd, 
Quart.  Journ.  Geol.  Soc.  xlii.  (1886),  p.  49.     G.  H.  Williams,  Bull.  U.  S.  GeoL 
Surv.  No.  28  (1886).     P.  D.  Chester,  op.  cit.  No.  59  (1890).     M.  E.  Wadaworth, 
GeoL  Surv.  Minnesota,  BulL  2,  1SS7. 


294  TEXT-BOOK   OF   GEOLOGY 

Gabbro  occurs  as  an  eruptive  rock  among  the  older  for- 
mations, likewise  in  large  bosses  and  dikes  in  volcanic  cores 
of  Tertiary  age  (Mull,  Skye).  Average  composition:  silica, 
49;  alumina,  15;  lime,  9 -5;  magnesia,  9-7;  oxides  of  iron 
and  manganese,  11-5;  potash,  0-3;  soda,  2-5.  Loss  by  igni- 
tion, 2*5;  specific  gravity,  2-85-3-10. 

T  he  following  varieties  may  be  noticed :  O  1  i  v  i  n  e-g  a  b- 
bro — a  granitoid  or  ophitic  compound  of  plagioclase,  au- 
gite, olivine,  and  magnetic  or  titaniferous  iron;  good  exam- 
ples are  found  among  the  deep-seated  parts  of  some  of  the 
Tertiary  volcanic  vents  of  the  Inner  Hebrides.  Hypers- 
thene-gabbro  or  Norite  (Hypersthenite,  Hyperite, 
Schillerfels) — with  a  rhombic  pyroxene  in  addition  to  or 
in  place  of  the  augite.  Troctolite  (Forellenstein) — a 
mixture  of  white  anorthite  with  dark-green  olivine,  re- 
ceives its  name  from  the  supposed  resemblance  of  its 
speckled  appearance  to  that  of  the  side  of  a  trout.  P  y- 
r  oxe  n  e-g  r  a  n  ul  i  t  e  (granular  diorite,  trap-granulite) — 
consisting  of  plagioclase,  pyroxene  (monoclimc  and  rhom- 
bic), hornblende,  and  garnet,  distinguished  by  the  granular 
condition  of  these  minerals,  and  found  among  gneisses  and 
other  schistose  rocks;  this  is  probably  an  altered  condition 
of  some  original  pyroxenic  eruptive  rock. 

Dolente — an  important  group  of  basic  rocks,  which  con- 
nect the  gabbros  with  the  basalts  and  include  many  of  the 
rocks  once  termed  "Greenstones."  They  are  composed  of 
labradorite  (or  anorthite),  with  some  ferro-magnesian  min- 
eral (augite,  enstatite,  olivine,  or  mica)  and  magnetic  or 
titaniferous  iron.  As  a  rule,  they  are  holocrystalline,  the 
constituent  felspar  and  pyroxene  or  olivine  being  character- 
istically grouped  in  ophitic  structure,  but  a  little  residual 
glass  may  occasionally  be  detected.  They  occur  in  bosses, 
intrusive  sheets,  and  dikes,  especially  as  the  subterranean 
accompaniments  of  the  volcanic  action  which  has  thrown 
out  augite-andesites  and  basalts  to  the  surface. 

Normal  or  ordinary  dolerite  consists  of  plagioclase  and 
augite,  with  magnetite  or  titanic  iron  and  frequently  olivine. 
Average  composition:  silica,  45-55;  alumina,  12-16;  lime, 
7-13;  magnesia,  3-9:  oxides  of  iron  and  manganese,  9-18; 
potash,  0-1;  soda,  2-5.  Loss  by  ignition  (water,  etc.),  0-5-3; 
specific  gravity,  2-75-2-96. 

Different  names  have  been  proposed  for  the  chief  varie- 
ties. The  most  important  of  these  are  0 1  i  v  i  n  e-d  o  1  e  r  i  t  e 
— a  dark,  heavy,  close-grained  finely-crystalline  rock,  with 
scattered  olivine,  apt  to  weather  with  a  brown  curst.  0 1  i- 


GEOGNOSY  290 

vine-free  dole  rite — a  similar  rock  but  containing  no 
olivine.  Enstatit  e-d  o  I  e  r  i  t  e  contains  enstatite  in  ad- 
dition to  the  other  ingredients.  N  e  p  h  e  1  i  n  e-d  o  1  e  r  i  t  e, 
has  the  felspar  largely  or  entirely  replaced  by  nepheliue  (see 
Nephelinite,  p.  299). 

As  varieties  of  dolerite  depending  for  their  peculiarities 
mainly  upon  their  antiquity  and  the  consequent  alteration 
they  have  undergone,  we  may  include  the  rocks  compre- 
hended under  the  term  Diabase.198  This  name  was  given 
to  certain  dark  green  or  black  eruptive  rocks  found  in  older 
geological  formations,  and  consisting  essentially  of  triclinic 
felspar,  augite,  magnetite  or  titaniferous  iron,  apatite,  some- 
times olivine,  usually  with  more  or  less  of  diffused  greenish 
chloritic  substances  (viridite)  which  have  resulted  from  the 
alteration  of  the  augite  or  olivine.  The  average  composi- 
tion of  typical  diabase  may  be  taken  to  be:  silica,  48-50; 
alumina,  16-0;  protoxide  of  iron,  12-15;  lime,  5-11;  mag- 
nesia, 4-6;  potash,  0-8-l*5;  soda,  3-4'5;  water,  l-5-2.  Spe- 
cific gravity  about  2'9.  There  is  generally  carbonic  acid 
present,  united  with  some  of  the  lime  as  a  decomposition 

Eroduct.  As  in  ordinary  dolerite,  gradations  may  be  traced 
•om  coarsely  crystalline  diabase197  into  exceedingly  fine- 
grained and  compact  varieties  (Diabase-aphanite),  which 
sometimes  assume  a  fissile  character  (Diabase-schiefer)  where 
they  have  been  subjected  to  crushing  or  cleavage.  Some 
kinds  present  a  porphyritic  structure,  and  show  dispersed 
crystals  of  the  component  minerals  (Diabase-porphyry,  Lab- 
rador-porphyry, Augite-porphyry);  or,  as  in  some  varieties 
of  diorite,  a  concretionary  arrangement  is  produced  by  the 
appearance  of  abundant  pea-like  bodies  of  a  compact  felsitic 
material,  imbedded  in  a  compact  or  finely  crystalline  ground- 
mass  (Variolite).  When  the  green  compact  ground-mass 
contains  small  kernels  of  carbonate  of  lime,  sometimes  in 
great  numbers,  it  is  called  Calcareous  aphanite  or  Calcaph- 
anite.  Sometimes  the  rock  is  abundantly  amygdaloidal. 
Though,  as  a  rule,  free  silica  does  not  occur  in  it,  some 
varieties  found  to  contain  this  mineral,  possibly  a  second- 
ary product,  have  been  distinguished  as  Quartz-diabase. 
The  presence  of  olivine  has  suggested  the  name  Olivine- 


194  The  student  will  find  in  the  Zeitschrift.  Deutsch.  G-eol.  Ges.  1874,  p.  1, 
an  important  memoir  by  Dathe  on  the  composition  and  structure  of  diabase. 
See  also  Zirkel's  "Microscop.  Petrog."  p.  97. 

191  Michel-LeVy.  Bull.  Soc.  Geol.  France,  3d  ser.  xi.  p.  282.  Geikie,  Trans. 
Roy.  Soc.  Edin.  xxix.  p.  487. 


296 


TEXT-BOOK    OF   GEOLOGY 


diabase  as  distinguished  from  the  normal  kinds  in  which 
this  mineral  is  absent.  A  variety  containing  hornblende  is 
termed  Proterobase.  Ophite,  a  variety  occurring  in  the  Pyre- 
nees, contains  diallage  and  epidote  (see  p.  212). 

Diabase  occurs  both  in  contemporaneous  beds  and  in  in- 
trusive dikes  and  sheets. 

Basalt198 — a  black,  extremely  compact,  apparently  homo- 
geneous rock,  which  breaks  with  a  splintery  or  conchoidal 
fracture,  and  in  which  the  component  minerals  can  only 
be  observed  with  the  microscope,  unless  where  they  are 
scattered  porphyritically  through  the  mass  (Fig.  32).  The 
minerals  consist  of  plagioclase 
(labradorite  or  anorthite),  pyrox- 
ene (usually  augite,  but  occasion- 
ally a  rhombic  form),  olivine, 
magnetite  or  titaniferous  iron. 
Many  years  ago,  Andrews  de- 
tected native  iron  in  the  basalt 
of  Antrim,  and  more  recently 
Nordenskiold  found  this  sub- 
stance abundantly  diffused  in  the 
basalt  of  Disco  Island,  occurring 
even  in  large  blocks  like  meteor- 
ites (ante,  p.  125).  The  ground- 
mass  of  basalt  presents  under  the 
microscope  traces  of  glass  in 
which  are  imbedded  minute  gran- 
ules, hairs,  needles,  and  mierolites 

gated  into  a  large  compound  of  felspar  and  augite.  The  pro- 
crystal.  The  black  specks  are  portion  or  this  base  varies  within 
wide  limits,  insomuch  that  while 
in  some  parts  of  a  basalt  it  so  preponderates  that  the  indi- 
vidual crystals  are  scattered  widely  through  it,  or  are  drawn 
out  into  beautiful  streaks  and  eddies  of  fluxion  structure,  in 
others  it  almost  disappears,  and  the  rock  then  appears  as  a 
nearly  crystalline  mass,  which  thus  graduates  into  dolerite 
and  basic  andesite.  The  component  minerals  frequently 
appear  porphyritically  dispersed,  especially  the  olivine,  the 
pale  yellow  grains  of  which  are  characteristic. 

198  On  basalt  rocks  see  Zirkel's  "Basaltgesteine,"  1870.  Boricky's  "Petro- 
graphische  Studien  en  den  Basaltgesteinen  Bohmens,"  in  Archiv  fur  Naturwisa. 
Landesdurchforschung  von  Bohmen,  ii.  1873.  Allport,  Q.  J.  Geol.  Soc.  xxx. 
p.  529.  Geikie,  Trans.  Roy.  Soc.  Edin.  xxix.  Mohl,  Nov.  Act.  Acad.  Leop. 
Carol,  xxxvi.  (1873),  p.  74;  Neues  Jahrb.  1873,  pp.  449,  824.  F.  Eichstadt  on 
Basalts  of  Scania,  Sveriges  Geol.  Undersok,  ser.  c.  No.  51,  1882.  E.  Svedmark, 
op.  cit.  No.  60,  1883. 


shaded  crystals  are  Olivine  con- 
siderably serpentinized :  the  nu- 
merous small  white  prisms  are 
Plagioclase.  A  few  Augite  prisms 
occur  which,  to  the  right  of  the 
centre  of  the  drawing,  are  aggre- 
gated into  a  large  compound 


GEOGNOSY  297 

Two  types  of  basalt  have  been  recognized  in  the  great 
basaltic  outbursts  of  Western  America:  (1)  the  porphyritic, 
consisting  of  a  glassy  and  microlitic  or  micro-crystalline 
ground-mass,  bearing  relatively  large  crystals  of  olivine, 
felspar,  and  occasionally  augite,  a  structure  showing  close 
relations  to  that  of  many  andesites;  (2)  the  granular  (in  the 
sense  in  which  that  term  is  used  by  Eosenbusch,  ante,  p.  177) 
— an  aggregate  of  quite  uniform  "grains,  composed  of  well- 
developed  plagioclase  and  olivine  crystals,  with  ill-defined 
patches  of  augite,  and  frequently  with  a  considerable  amount 
of  glass-base.  By  diminution  of  olivine  and  augmentation 
of  silica,  and  the  appearance  of  hypersthene,  gradations  can 
be  traced  from  true  olivine-basalts  into  normal  andesites. 
Basalts  with  free  quartz  are  not  infrequent  in  Western 
America."* 

Basalt  occurs  in  amorphous  and  columnar  sheets,  which 
may  alternate  with  each  other  or  with  associated  tuffs.  It 
also  forms  abundant  dikes,  veins,  and  intrusive  bosses.  It 
frequently  assumes  a  cellular  structure,  which  becomes 
amygdaloidal  by  the  deposit  of  calcite,  zeolites,  or  other 
minerals  in  the  vesicles.  A  relation  may  be  traced  between 
the  development  of  amygdales  and  the  state  of  the  rock;  the 
more  amygdaloidal  the  rock,  the  more  is  it  decomposed, 
showing  that  the  amygdales  have  probably  in  large  meas- 
ure been  derived  by  infiltrating  water  from  the  basalt  itself. 

Yitreous  Basalt  (Basalt-glass,  Tachylyte,  Hyalo- 
melan).200 — Basalt  passes  into  a  condition  which,  even  to 
the  naked  eye,  is  recognizable  as  that  of  a  true  glass.  This 
more  especially  takes  place  along  the  edges  of  dikes  and. 
intrusive  sheets.  Where  an  external  skin  of  the  original 
molten  rock  has  rapidly  cooled  and  consolidated,  in  contact 
with  the  rocks  through  which  the  eruption  took  place,  a 
transition  can  be  traced  within  the  space  of  less  than  a  quar- 
ter of  an  inch  from  a  crystalline  dolerite,  anamesite,  basalt, 
or  andesite  into  a  black  glass,  which  under  the  microsqope 
assumes  a  pale  brown  or  yellowish  color,  and  is  isotropic, 
but  generally  contains  abundant  microlites,  sometimes  with 
a  globular  or  spherulitic  concretionary  structure.  In  such 
cases  it  seems  indisputable  that  this  glass  represents  what 


199  Hague  and  Iddings,  Amer.  Journ.  Sci.  xxvii.  (1884),  p.  456.  Iddings, 
op.  cit.  xxxvi.  (1888),  p.  208,  Bull.  U.  S.  Geol.  Surv.  Noa.  66  and  79  (J.  S.  Diller). 

400  See  Judd  &  Cole,  Q.  J.  Geol.  Soc.  xxxix.  (1883),  p.  444.  Cole,  op.  cit. 
xliv.  (1888),  p.  300.  Cohen,  Neues  Jahrb.  1876,  p.  744;  1880  (vol.  ii.),  p.  23 
(Sandwich  Islands). 


298  TEXT-BOOK   OF   GEOLOGY 

was  the  general  condition  of  the  whole  molten  mass  at  the 
time  of  eruption,  and  that  the  present  crystalline  structure 
of  the  rock  was  developed  during  cooling  and  consolidation. 
The  glassy  forms  of  basalt  undergo  alteration  into  a  yellow- 
ish substance  called  Palagonite  (p.  242).  It  is  worthy 
of  remark  that  in  the  analyses  of  vitreous  basalts,  the  per- 
centage of  silica  rises  usually  above,  while  their  specific 
gravity  falls  below,  that  of  ordinary  crystalline  basalt. 

The  average  composition  of  basalt  is — silica,  45-55; 
alumina,  10-18;  lime,  7-14;  magnesia,  3-10;  oxides  of 
iron  and  manganese,  9-16;  potash,  0-5-3;  soda,  2-5.  Loss 
by  ignition  (water,  etc.),  l-x>;  specific  gravity,  2'85-3'10. 

The  basalt-rocks  are  thoroughly  volcanic  in  origin,  ap- 
pearing in  lava-streams,  plateaus,  sills,  necks,  dikes,  and 
veins.  The  columnar  structure  is  so  common  among  the 
finer-grained  varieties  that  the  term  "basaltic"  has  been 
popularly  used  to  denote  it.  As  already  stated,  it  has  been 
assumed  by  some  writers  that  basalt  did  not  begin  to  be 
erupted  until  the  Tertiary  period.  But  true  basalt  occurs 
abundantly  in  Scotland  as  a  product  of  Lower  Carboniferous 
volcanoes,  and  exhibits  there  a  variety  of  types  of  minute 
structure."01 

Basic  Pumice. — Though  the  acid  lavas  furnish  most 
of  the  pumice  with  which  we  are  familiar,  some  of  the  basic 
kinds  also  assume  a  similar  structure.  Thus  at  Hawaii,  the 
basic  pyroxenic  or  olivine  lavas  give  rise  to  a  pumiceous 
froth. 

Melaphyre — a  name  originally  proposed  by  Brong- 
niart  and  subsequently  applied  in  various  senses  by  different 
writers  to  include  rocks  which  range  in  structure 'and  corn- 
position  from  the  more  basic  andesites  to  true  olivine- basalts. 
The  melaphyres  for  the  most  part  belong  to  pre-Tertiary 
eruptions  (though  some  Tertiary  lavas  have  been  described 
as  melaphyre)  and  have  undergone  more  or  less  alteration. 
If  the  word  is  to  be  retained  as  a  definite  rock- name  it  should 
be  restricted  to  an  altered  type,  as  is  now  generally  agreed, 
and  preferentially  to  the  older  altered  basalts.  The  mela- 
phyres will  then  bear  somewhat  the  same  relation  to  the 
basalts  that  the  diabases  do  to  the  dolerites  and  the  porphy- 
rites  to  the  andesites.  But  it  must  necessarily  happen  that 
difficulty  will  be  experienced  in  deciding  which  of  the  three 

201  See  Trans.  Boy.  Soc.  Edin.  xxix.  <1879),  p.  437,  and  Presidential  Address, 
Quart.  Journ.  Geol.  Soc.  (1892),  p.  129,  where  the  types  of  microscopic  structure 
observed  by  Dr.  Hatch  are  enumerated. 

\ 


GEOGNOSY  299 

names  would  be  best  applied  to  some  of  the  eruptive  rocks 
of  the  older  geological  formations.  The  melaphyres,  as  thus 
defined,  are  somewhat  dull,  dark  brown,  reddish,  or  green 
rocks,  often  amygdaloidal  and  showing  their  porphyritic 
minerals  in  an  altered  condition,  the  olivines  especially 
being  changed  into  serpentine  or  replaced  by  magnetite  or 
even  by  haematite.2011 

Nepheline-basalt  (Nepheline-Basanite).  — •  Zirkel 
proved  that  certain  black  heavy  rocks,  having  externally 
the  aspect  of  ordinary  basalt,  contain  little  or  no  felspar, 
the  part  of  that  mineral  being  taken  in  some  by  nephelme, 
in  others  by  leucite.*08  They  are  volcanic  masses  of  late 
Tertiary  age,  but  occur  much  more  sparingly  than  the  true 
basalts.  They  are  found  in  the  Odenwald,  Thuringer  Wuld, 
Brzgebirge,  Baden,  etc.  Mean  composition — silica,  45'52; 
alumina,  16*50;  ferric  and  ferrous  oxides,  11*20;  lime, 
10-62;  magnesia,  4*35;  potash,  1*95;  soda,  5*40;  water, 
2*68.  Mean  specific  gravity,  2-9-3-1.  Nephelinite  is 
a  form  of  basalt  with  no  felspar  or  olivine. 

L  e  u  c  i  t  e-b  a  s  a  1 1  (Leucite-Basanite)  contains  little  or 
no  felspar,  but  has  leucite  in  place  of  it.  Externally  it 
resembles  ordinary  basalt.  This  rock  occurs  among  the 
extinct  volcanoes  of  the  Bifel  and  of  Central  Italy,  and 
forms  the  lavas  of  Vesuvius.  Leucitite  contains  no  fel- 
spar and  no  olivine. 

M  el  i  lite-basalt. — In  continuation  of  Zirkel's  re- 
search, A.  Stelzner  has  shown  that  in  some  basults  the 
part  of  felspar  and  nepheline  is  played  by  melilite. *04  In 
outer  appearance  the  rocks  possessing  this  composition,  and 
to  which  the  name  of  Melilite-basalt  has  been  given,  cannot 
be  distinguished  from  ordinary  basalt.  Under  the  micro- 
scope, the  ground-mass  appears  to  be  mainly  composed  of 
transparent  sections  of  melilite,  either  disposed  without 
order,  or  ranged  in  fluxion  lines  round  the  large  olivine 
and  augite  crystals;  but  it  also  contains  chromite  (?),  niicro- 
litic  augite,  brown  mica,  abundant  magnetite,  with  perow- 
skite,  apatite,  and  probably  nepheline.  (Swabian  Alb, 
Bohemia,  Saxon  Switzerland,  etc.) 


*°J  For  some  account  of  the  use  of  the  word  melaphyre  see  Brongniart, 
"Classification  et  Caracteres  mineralotriques  des  Roches  homogenes  et  hetero- 
genes,"  1827,  p.  106.  Naumann,  "Lehrbuch  der  Geognosie,"  i.  p.  587.  Zir- 
kel, "Petrographie, "  ii.  p.  39.  Rosenbusch,  "Mikroskop.  Physiogr."  ii.  p.  484. 

903  "Basaltgesteine,"  1870. 

404  Neues  Jahrb.  (Beilageband),  1883,  p.  369-439. 


300  TEXT-BOOK   OF   GEOLOGY 

Under  the  awkward  name  of  "ultra-basic,"  the  follow- 
ing group  of  rocks  is  included  in  which  the  proportion  of 
silica  sinks  to  a  still  smaller  amount  than  in  the  basalts. 

Ltmburgtte  (Magma-basalt) — a  fine-grained  to  vitreous  rock 
composed  of  augite,  olivine,  magnetite  or  titaniferous  iron, 
and  apatite.  The  base  is  generally  glassy  and  the  propor- 
tion of  silica  in  the  rock  is  only  about  42  per  cent.  The 
typical  locality  is  Limburg,  near  the  Kaiserstuhl  in  Baden. 

Peridotite  Group. — The  rocks  here  embraced,  stand  at  the 
extreme  end  of  the  basic  igneous  rocks  as  the  rhyolites  and 
granites  stand  at  the  opposite  end  of  the  acid  series.  They 
contain  no  felspar,  or  at  least  an  insignificant  proportion  of 
it,  and  consist  of  olivine,  with  augite,  hornblende  or  mica, 
magnetic  or  titaniferous  iron,  chromite  and  other  allied 
minerals  of  the  spinel  type.  They  contain — silica,  39-i5; 
alumina,  0-6;  ferrous  oxide,  8-10;  lime,  0-2;  magnesia, 
35-48;  and  have  a  mean  specific  gravity  between  3-0  and 
3 -3.  When  quite  fresh  these  rocks  have  a  holocrystalline 
structure,  but  they  are  generally  more  or  less  altered,  and 
in  their  extreme  condition  of  alteration  form  rocks  known 
as  serpentines.  They  occur  for  the  most  part  as  intrusive 
masses  belonging  to  the  deeper-seated  portions  of  volcanic 
eruptions.  The  following  varieties  may  be  noticed: 

Pikrite206  (Palaeopikrite,  Pikrite-porphyry) — a  rock 
rich  in  olivine,  usually  more  or  less  serpentinized,  with 
augite,  magnetite,  or  ilmenite,  brown  biotite,  hornblende, 
or  apatite;  occurs  as  an  eruptive  rock  among  Palaeozoio 
formations;  is  closely  related  to  the  diabases  into  which 
by  the  addition  of  plagioclase  it  naturally  passes.  When 
hornblende  predominates  over  pyroxene  the  rock  has  been 
called  hornblende-pikrite. 

Lh erzo li  teaoe — so  named  from  L'herz  in  the  Ariege, 
is  a  holocrystalline  rock  composed  of  olivine,  diallage,  and 
a  rhombic"  pyroxene,  with  a  lesser  proportion  of  a  spinel- 
loid  sometimes  brown  (chromite,  picotite),  sometimes  green 
(pleonast),  and  iron  ores. 

D  u  n  i  t  e,  named  by  F.  von  Hochstetter  from  the  Dun 
Mountain,  New  Zealand,  consists  of  a  granitoid  mixture  of 
olivine  with  chromite  or  other  spinelloid.  Such  a  rock 
passes  naturally  by  alteration  into  a  serpentine. 

905  So  named  from  mtpfc,  bitter,  in  allusion  to  the  large  proportion  of  bitter- 
earth  (Magnesia) — a  character  shared  by  all  the  peridotites.  GiimbeJ,  "Die 
Palaeolithischen  Eruptivgesteine  des  Fichtelgebirges" ;  Munich,  1874. 

206  On  the  eruptive  nature  of  Lherzolite,  see  A.  Lacrois,  Compt.  rend.  civ. 
(1892),  pp.  974  and  976. 


GEOGNOSY  301 

Serpentine.9" — Under  this  name  are  included  rocks  which, 
whatever  may  have  been  their  original  character  and  com- 
position, now  consist  mainly  or  wholly  of  serpentine.  As 
already  stated,  olivine  readily  passes  into  the  condition  of 
serpentine,  while  the  other  minerals  may  remain  nearly  un- 
affected, as  is  admirably  seen  in  some  pikrites.  Most  ser- 
pentine-rocks originally  consisted  principally  of  olivine 
(see  Fig.  33).  Diorite,  gabbro,  and  other  rocks,  consisting 
largely  of  magnesian  silicates,  likewise  pass  into  serpen- 
tine. "  If  varieties  due  to  different  phases  of  alteration  were 
judged  worthy  of  separate  designation,  each  member  of  the 
peridotites  might  of  course  have  a  conceivable  or  actual 
representative  among  the  serpentines.  But  without  attempt- 


Fig.  83. -Stages  in  the  alteration  of  Olivine.    A,  the  nearly  fresh  crystal;  B,  the 
alteration  half  completed;  c,  the  crystal  wholly  serpentinized. 

ing  this  minuteness  of  classification,  we  may  with  advantage 
treat  by  itself,  as  deserving  special  notice,  the  massive  form 
of  the  mineral  serpentine  from  whatsoever  rock  it  may  have 
originated. 

Massive  serpentine  is  a  compact  or  finely  granular,  faintly 
glimmering,  or  dull  rock,  easily  cut  or  scratched,  having  a 
prevailing  dirty-green  color,  sometimes  variously  streaked 
or  flecked  with  brown,  yellow,  or  red.  It  frequently  con- 
tains other  minerals  besides  serpentine.  One  of  its  com- 
monest accompaniments  is  chrysotile  or  fibrous  serpentine, 
which  in  veinings  of  a  silky  lustre  often  ramifies  through 
the  rock  in  all  directions.  Other  common  inclosures  are 

201  See  Tschermak,  Sitz.  Akad.  Wien,  Ivi.  July,  1867;  it  was  this  author 
who  first  showed  the  derivation  of  serpentine  from  original  olivine  rocks ;  Bon- 
ney,  Q.  J.  Geol.  Soc.  xxxiii.  p.  884,  xxxiv.  p.  769;  Geol.  Mag.  (2)  vi.  p.  362; 
(3)  i.  p.  406;  Michel-LeVy,  Bull.  Soc.  Ge"ol.  France,  vi.  3d  ser.  p.  156;  Sterry 
Hunt,  Trans.  Roy.  Soc.  Canada,  i.  (1883);  Dathe,  Neues  Jahrb.  1876,  pp.  236, 
337,  where  Garnet-serpentine  and  Bronzite-serpentine  are  described  from  the 
Saxon  granulite  region.  J.  S.  Diller,  Bull.  U.  S.  Geol.  Burv.  No.  38  (1887); 
M.  E.  Wadsworth,  "Lithological  Studies"  (1884),  p.  118. 


302  TEXT-BOOK    OF   GEOLOGY 

bronzite,  enstatite,  magnetite,  and  chrome-spinels,  besides 
traces  of  the  original  olivine,  pyroxene,  amphibole,  mica, 
or  felspar  in  the  rocks,  which  have  been  altered  into 
serpentine. 

Serpentine  occurs  in  two  distinct  forms;  1st,  in  beds  or 
bands  intercalated  among  schistose  rocks,  and  associated 
especially  with  crystalline  limestones;  2dly,  in  dikes,  veins, 
or  bosses  traversing  other  rocks. 

As  to  its  mode  of  origin,  there  can  be  no  doubt  that  in 
most  cases  it  was  originally  an  eruptive  rock,  as  is  clearly 
shown  by  its  occurrence  in  dikes  and  irregular  bosses.  The 
frequent  occurrence  of  recognizable  olivine  crystals,  or  of 
their  still  remaining  contours,  in 
the  midst  of  the  serpentine-matrix, 
affords  good  grounds  for  assigning 
an  eruptive  origin  to  many  serpen- 
tines which  have  no  distinctly  erup- 
tive external  form  (Fig.  34).  The 
rock  cannot,  of  course,  have  been 
ejected  as  the  hydrous  magnesian 
silicate  serpentine;  we  must  regard 
it  as  having  been  originally  an 
eruptive  olivine  rock,  or  a  highly 
hornblendic  or  micaceous  diorite, 
F\g.  34.-Microscopic  structure  of  or  olivine-gabbro.  But,  on  the 

Serpentine  (20  D.ameters).  ^      intercalation      Qf 


beds  of  serpentine  among  schistose  rocks,  and  particularly 
the  frequent  occurrence  of  serpentine  in  connection  with 
more  or  less  altered  limestones  (West  of  Ireland,  High- 
lands of  Scotland)  suggests  another  mode  of  origin  in  these 
cases.  Some  writers  have  contended  that  such  serpentines 
are  products  of  the  alteration  of  dolomite,  the  magnesia 
having  been  taken  up  by  silica,  leaving  the  carbonate  of 
lime  behind  as  beds  of  limestone.  Others  have  supposed 
the  original  rocks,  from  which  the  serpentines  were  derived, 
to  have  been  a  deposit  from  oceanic  water,  as  has  been  sug- 
gested by  Sterry  Hunt  in  the  case  of  those  associated  with 
the  crystalline  schists.208  Beds  of  serpentine  intercalated  with 
limestone  might  conceivably  have  been  due  to  the  elimina- 
tion of  magnesian  silicates  from  sea-  water  by  organic  agency, 
like  the  glauconite  now  found  filling  the  chambers  of  Jo- 
raminifera,  the  cavities  of  corals,  the  canals  in  shells,  sea- 
urchin  spines  and  other  organisms  on  the  floor  of  the  present 

*°8  "Chemical  Essays,"  p.  123. 


GEOGNOSY  303 

sea."09  Among  the  limestone  and  crystalline  schists  of 
Banffshire  (p.  316),  serpentine  occurs  in  thick  lenticular 
beds  which  possess  a  schistose  crumpled  structure  and 
agree  in  dip  with  the  surrounding  roc£s.  They  may  have 
been  deposits  of  contemporaneous  origin  with  the  lime- 
stones and  schists  among  which  they  occur,  and  in  asso- 
ciation with  which  they  have  undergone  the  characteristic 
schistose  puckering  and  crumpling.  Sometimes  they  sug- 
gest a  source  from  the  alteration  of  highly  basic  volcanic 
tuffs.  In  other  cases  they  may  have  been  erupted  peridotites 
which  have  acquired  a  schistose  character  from  the  same 
process  of  mechanical  deformation  that  has  played  so  large 
a  part  in  producing  the  foliation  of  the  crystalline  schists. 

III.  SCHISTOSE  (METAMORPHIC) 

In  this  section  is  comprised  a  series  of  rocks  which 
present  a  remarkable  system  of  divisional  planes  that  are 
not  original  but  have  been  superinduced  upon  them.  At 
the  one  end  stand  rocks  which  are  unmistakably  of  sedi- 
mentary origin,  for  their  original  bedding  can  often  be  dis- 
tinctly seen,  and  they  also  contain  organic  remains  similar 
to  those  found  in  ordinary  unaltered  sedimentary  strata. 
At  the  other  end  come  coarsely  crystalline  masses,  which 
in  many  respects  resemble  granite,  and  the  original  char- 
acter of  which  is  not  obvious.  An  apparently  unbroken 
gradation  can  be  traced  between  these  extremes,  and  the 
whole  series  has  been  termed  "metamorphic"  from  the 
changed  form  in  which  its  members  are  believed  now  to 
appear.  In  the  earlier  stages  the  change  has  taken  the  form 
of  cleavage  as  in  ordinary  slate.  Even  in  slate,  however, 
as  already  remarked  (p.  236),  a  beginning  may  be  detected 
in  the  development  of  crystalline  particles,  and  the  crystal- 
line re-arrangement  may  be  traced  in  constantly  advancing 


209  According  to  Berthier,  one  of  the  glauconitic  deposits  in  a  Tertiary  lime- 
stone is  a  true  serpentine.     See  Sterry  Hunt,  "Chem.  Essays,"  p.  303. 


304  TEXT-BOOK    OF    GEOLOGY 

progression   until   the  whole  mass   has  become  crystalline, 
and  forms  what  is  known  as  a  schist. 

The  Crystalline  Schists,  properly  so  called,  constitute  a 
well-defined  series  of  rocks.  They  are  mainly  composed 
of  silicates.  Their  structure  is  crystalline,  but  is  distin- 
guished from  that  of  the  Massive  or  Eruptive  rocks  by  its 
more  or  less  closely  parallel  layers  or  folia,  consisting  of 


Fig.  35.— Profile  of  a  piece  of  Gneiss,  showing  the  lenticular  character  of 
its  folia,  natural  size.    (B.  N.  Peach.) 

materials  which  have  assumed  a  crystalline  character  along 
these  layers.  The  folia  may  be  composed  of  only  one  min- 
eral, but  usually  consist  of  two  or  more,  which  occur  either 
in  distinct,  often  alternate  laminae,  or  intermingled  in  the 
same  layer.  This  structure  resembles  that  of  the  stratified 
rocks,  but  it  is  differentiated  (1)  by  a  prevalent  striking 
want  of  continuity  in  the  folia,  which,  as  a  rule,  are  con- 
spicuously lenticular,  thickening  out  and  then  dying  away, 


GEOGNOSY  805 

and  reappearing  after  an  interval  on  the  same  or  a  different 
plane  (Fig.  35);  (2)  by  a  peculiar  and  very  characteristic 
welding  of  the  folia  into  each  other,  the  crystalline  par- 
ticles of  one  layer  being  so  intermingled  with  those  of  the 
layers  above  and  below  it  that  the  whole  coheres  as  a  tough, 


Pig.  86.— View  of  a  hand-specimen  of  contorted  mica-schtot,  two-thirds  natural 
size.    (B.  N.  Peach.) 

not  easily  fissile  mass;  (3)  by  a  frequent  remarkable  and 
eminently  distinctive  puckering  or  crumpling  (with  frequent 
minute  faulting)  of  the  folia,  which  becomes  sometimes  so 
fine  as  to  be  discernible  only  under  the  microscope118  (Fig. 


210  On  the  microscopic  structure  of  the  crystalline  schists  see  Zirkel,  "Micro- 
scopical Petrography"  (vol.  vi.  of  King's  Exploration  of  40th  Parallel),  1876, 
L14.     Allport,  Q.  J.  Geol.  Soc.  xxxii.   p.  407.     Sorby,  op.  cit.  xxxvi.  p.  81, 
limann's  "Untersuchungen  uber  d.  Entstehung.  Altkryst.  Schiefer,"  Bonn, 
1884;  and  other  memoirs  ciled  in  subsequent  pages. 


306  TEXT-BOOK    OF   GEOLOGY 

87),  but  is  often  present  conspicuously  in  hand-specimens 
(Fig.  36),  and  can  be  traced  in  increasing  dimensions,  till 
it  connects  itself  with  gigantic  curvatures  of  the  strata, 
which  embrace  whole  mountains.  These  characters  are 
sufficient  to  indicate  a  great  difference  between  schistose 
rocks  and  ordinary  stratified  formations,  in  which  the  strata 
lie  in  continuous  flat,  parallel,  and  more  or  less  easily 
separable  layers. 

In  some  instances,  the  folia  can  be  seen  to  coincide  with 
original  bedding,  as  where  a  band  of  quartzite  or  of  con- 
glomerate is  intercalated  between  sheets  of  phyllite  or  mica- 
schist.  In  such  cases,  there  cannot  be  any  doubt  that  the 
rock,  though  now  more  or  less  reconstructed  and  crystal- 
line, was  originally  mere  accumulated  mechanical  sediment. 
Many  clay  slates,  phyllites,  and  mica-schists  are  obviously 
only  altered  marine  clays,  and  some  of  them  still  retain  their 
recognizable  fossils.  From  such  rocks,  gradations  can  be 
followed  into  chiastolite-schist,  mica-schist,  and  fine  gneiss. 
Quartzites  and  quartz-schists  often  still  retain  the  false-bed- 
ding of  the  original  sandy  sediment  of  which  they  are  com- 
posed. The  pebbly  and  conglomeratic  bands  associated  with 
some  schists  afford  convincing  proof  of  their  original  clastic 
nature.  Thus,  at  the  one  end  of  the  schistose  series  we  find 
rocks  in  which  an  original  sedimentary  character  remains 
unmistakable.  At  the  other  end,  after  many  intermediate 
stages,  we  encounter  thoroughly  amorphous  crystalline 
masses,  that  bear  the  closest  resemblance  to  eruptive  rocks 
into  which  they  insensibly  pass.  In  such  instances,  there 
can  be  little  doubt  that  the  amorphous  structure  is  the  origi- 
nal one,  which  has  become  schistose  by  subsequent  defor- 
mation (Book  IV.  Part.  VIII.).  The  banded  arrangement 
of  many  coarse  gneisses,  however,  may  be  an  original  segre- 


GEOGNOSY  307 

gation-stnicture,  like  that  observable  in  sills  and  bosses  of 
eruptive  rocks  (p.  1035). 

In  the  more  thoroughly  reconstructed  and  recry stall ized 
schists  all  trace  of  the  original  structures  has  been  lost. 
The  foliation  is  not  coincident  with  bedding,  nor  with  any 
structure  of  eruptive  rocks,  but  has  been  determined  by 
planes  of  cleavage  or  of  shearing,  or  by  the  alignment  as- 
sumed by  minerals  crystallizing  under  the  influence  of  in- 
tense pressure.  Along  these  surfaces  the  constituents  have 
rearranged  themselves,  and  new  chemical  and  mineralogical 
combinations  have  been  effected  during  the  progress  of  the 
' '  metamorphism. ' ' 

A  rock  possessing  a  crystalline  arrangement  into  sepa- 
rate folia  is  in  English  termed  a  Schist.811  This  word, 
though  employed  as  a  general  designation  to  describe  the 
structure  of  all  truly  foliated  rocks,  is  also  made  use  of  as 
a  suffix  to  the  names  of  the  minerals  of  which  some  of  the 
foliated  rocks  largely  consist.  Thus  we  have  "mica-schist," 
"chlorite-schist, "  "hornblende-schist."  If  the  mass  loses 
its  fissile  tendency,  owing  to  the  felting  together  of  the  com- 
ponent mineral  into  a  tough  coherent  whole,  the  word  rock 
is  usually  substituted  for  schist,  as  in  "hornblende-rock," 
"actinolite-rock,"  and  so  on.  The  student  must  bear  in 
mind  that  while  the  possession  of  a  foliated  structure  is  the 
distinctive  character  of  the  crystalline  schists,  it  is  not  al- 
ways present  in  every  individual  bed  or  mass  associated 
with  these  rocks.  Yet  the  non-schistose  portions  are  so  ob- 
viously integral  parts  of  the  schistose  series  that  they  can- 
not, without  great  violation  of  natural  affinities,  be  separated 

211  In  French  this  term  has  no  such  definite  signification,  being  applied  both 
to  schists  and  to  shales.  In  German  also  the  corresponding  word  "schiefer" 
designates  schists,  but  is  also  employed  for  non-crystalline  shaly  rocks;  thon- 
schiefer  =  clay-slate :  schieferthon  =  shale. 


308  TEXT-BOOK    OF   GEOLOGY 

from  them.  Hence  in  the  following  enumeration  they  are 
included  as  common  accompaniments  of  the  schists.  Quart- 
zite  also  may  be  placed  in  this  subdivision,  though  in  its 
typical  condition  it  shows  no  schistose  structure. 

The  origin  of  the  crystalline  schists  has  been  the  subject 
of. long  discussion  among  geologists.  Werner  held  that, 
like  other  rocks  of  high  antiquity,  they  were  chemical  pre- 
cipitates from  a  universal  ocean.  Hutton  and  his  followers 
maintained  that  they  were  mechanical  aqueous  sediments 
altered  by  subterranean  heat.  These  two  doctrines  in  vari- 
ous modifications  are  still  maintained  by  opposite  schools. 
In  recent  years  much  light  has  been  thrown  upon  the  origin 
of  the  schistose  structure,  which  has  been  shown  to  be  in 
many  cases  due  to  the  mechanical  crushing  and  chemical 
readjustment  and  recrystallization  of  the  materials  of  both 
sedimentary  and  igneous  rocks.  This  subject  is  discussed 
in  a  later  part  of  this  work.  (See  Book  IV.  Part  VIII.) 

It  is  obvious  that  a  wide  series  of  rocks  embracing  vari- 
ously altered  forms  of  both  sedimentary  and  igneous  mate- 
rials hardly  admits  of  any  simple  system  of  classification. 
Regarding  them  from  the  point  of  view  of  the  nature  of  the 
metamorphism  they  have  undergone,  geologists  have  some- 
times grouped  these  rocks  as  resulting  either  from  contact 
metamorphism,  that  is,  from  the  effects  of  the  protrusion  of 
igneous  matter  from  within  the  earth's  interior,  or  from  re- 
gional metamorphism  where  the  changes  have  been  brought 
about  by  some  widespread  terrestrial  disturbance  (Book  IV. 
Part  VIII.).  But  this  arrangement,  though  of  value  in  dis- 
cussing questions  of  metamorphism,  has  the  disadvantage  of 
introducing  theoretical  considerations,  and  of  placing  in  dif- 
ferent groups  rocks  which  undoubtedly  present  the  same 
general  petrographical  characters.  Avoiding  all  disputed 


GEOGNOSY  309 

questions  as  to  modes  of  origin,  I  shall  group  the  schists 
according  to  their  mineral  characters,  beginning  with  those 
which  are  obviously  only  a  further  stage  of  the  alteration  of 
clay-slates,  and  ending  with  the  gneisses,  which  bear  a  close 
affinity  to  granites. 

1.  ARGILLITES,  ARGILLACEOUS  SCHISTS,  PHYLLITES. — 
The  rocks  included  in  this  group  may  often  be  traced  into 
the  clay-slates  described  on  p.  235.  They  mark  a  further 
stage  of  metamorphism,  wherein  besides  mechanical  defor- 
mation there  has  been  a  more  or  less  decided  recrystalliza- 
tion  of  the  materials,  which  is  demonstrated  by  the  abundant 
secondary  mica  and  by  the  appearance  of  such  minerals  as 
chiastolite,  andalusite,  staurolite,  garnet,  etc.  When  a  clay- 
slate  becomes  lustrous  by  the  development  of  mica,  it  is 
known  as  P  h  y  1 1  i  t  e — a  term  which  may  be  regarded  as 
embracing  the  intermediate  group  of  rocks  between  normal 
clay-slates  and  true  mica-schists. 

'Chiastolite-slate  (schiste  made),  a  clay-slate  i n 
which  crystals  of  chiastolite  have  been  developed,  even 
sometimes  side  by  side  with  still  distinctly  preserved  grap- 
tolites  or  other  organic  remains8"1  (Skiddaw,  Aberdeenshire, 
Brittany,  the  Pyrenees,  Saxony,  Norway,  Massachusetts, 
etc.).  Staurolite-slate,  a  micaceous  clay-slate  with 
crystals  of  staurolite  (Banffshire,  Pyrenees).  Ottrelite- 
slate,  a  clay-slate  marked  by  minute,  six-sided,  grayish 
or  blackish  green  lamellae  of  ottrelite  (Ardennes,  where  it  is 
said  to  contain  remains  of  trilobites,  Bavaria,  New  England). 
Di pyre-slate  is  full  of  small  crystals  of  dipyre.  Seri- 
ci  t  e-p  hy  Hi  te  is  a  name  proposed  by  Lossen  for  those 
compact,  greenish,  reddish,  or  violet  sericite-schists  in  which 
the  naked  eye  can  no  longer  distinguish  the  component  min- 
erals. M  i  c  a-p  h  y  1 1  i  t  e  (phyllade  gris  feuiltete  of  Du- 
mont),  a  silky,  usually  very  fissile  slate,  with  minute  scales 
of  mica.  German  petrographers  have  distinguished  by  name 
some  other  varieties  found  in  metamorphic  areas  and  char- 
acterized by  different  kinds  of  concretions,  but  to  which  no 


212  A  good  illustration  of  this  association  is  figured  by  Kjerulf  in  his  "Geo- 
logie  des  Siidlichen  und  Mittleren  Norwegen,"  Plate  xiv.  fig.  246.  See  also 
Brogger's  memoir  on  Upper  Silurian  fossils  among  the  crystalline  rocks  of  Ber- 
gen. Christiania,  1882.  A  similar  association  occurs  in  the  graptolite-shalea 
next  the  granite  of  Galloway,  Scotland. 


310  TEXT-BOOK    OF   GEOLOGY 

special  designations  have  been  given  in  English.  K  n  o- 
tense hiefer  (Knotted  schist)  contains  little  knots  or  con- 
cretions of  a  dark-green  or  brown,  fine-granular,  faintly 
glimmering  substance,  of  a  talcose  or  micaceous  nature,  im- 
bedded in  a  finely-laminated  matrix  of  a  talc-like  or  mica- 
like  mineral."18  These  aggregations  appear  to  be  in  many 
cases  incipient  stages  in  the  formation  ot  definite  crystals  of 
such  minerals  as  andalusite.  In  Fruchtschiefer  the 
concretions  are  like  grains  of  corn ;  in  Garbenschiefer, 
like  caraway  seeds ;  in  Fleckschiefer,  like  flecks  or 
spots.  Some  of  these  rocks  might  be  included  with  the 
mica-schists,  into  varieties  of  which  they  seem  to  pass. 
Eound  some  of  the  eruptive  diabase  of  the  Harz,  the  clay- 
slates  have  been  altered  into  various  crystalline  masses  to 
which  names  have  been  attached.  Tbus  Spilosite  is  a 
greenish,  schistose  rock,  composed  of  finely  granular  or  com- 
pact felspathic  material,  with  small  chlorite  concretions  or 
scales.  Desmosite  is  a  schistose  mass  in  which  similar 
materials  are  disposed  in  more  distinct  alternations.814 

2.  QUARTZ  KocKS."16 — Quartz-schist  (schistose  quartzite), 
an  aggregate  of  granular  (or  granulitic)  quartz  with  a  suffi- 
cient development  of  fine  folia  of  mica  to  impart  a  more  or 
less  definitely  schistose  structure  to  the  rock.  The  disap- 
pearance of  the  mica  gives  quartzite,  and  the  greater  promi- 
nence of  this  mineral  affords  gradations  into  mica-schist. 
Such  gradations  are  quite  analogous  to  those  among  recent 
sedimentary  materials  from  pure  sand,  through  muddy  sand, 
and  sandy  mud,  into  mud  or  clay,  and  between  sandstones 
and  shales.  The  Highlands  of  Scotland,  for  instance,  em- 
brace large  tracts  of  quartz-schists-rocks  which  are  not  prop- 
erly either  mica-schist  or  ordinary  quartzite.  They  consist 
of  granular  (granulitized)  quartz,  with  fine  parallel  laminae 
of  mica,  and  are  capable  of  being  split  into  thick  or  thin 
flagstones.  Interstratified  pebbly  varieties  occur. 

Itacolumite — a  schistose  quartzite,  in  which  the 
quartz-granules  are  separated  by  fine  scales  of  mica,  talc, 

418  A,  von  Lasaulx,  Neues  Jahrb.  1872,  p.  840.  K.  A.  Lossen,  Z.  Deutsch, 
Geol.  Ges.  1867,  p.  585  (where  a  detailed  description  of  the  Taunus  phyllitea 
will  be  found),  1872,  p.  757. 

814  Other  names  are  Bandschiefer,  Contactschiefer,  etc.  See  K.  A.  Lessen, 
Zeitach.  Deutsch.  Geo.  G«s.  xix.  (1867),  p.  509,  xxi.  p.  291,  xxiv.  p.  701. 
Kayser,  op.  cit  xxii.  p.  103. 

"•  J.  Maculloch,  Trans.  Geol.  Soc.  1st  ser.  ii.  (1814),  p.  450,  iv.  {1817),  p. 
264;  2d  ser.  i.  (1819),  p.  53.  Lossen,  Zeitsch.  Deutsch.  Geol.  Ges.  lix.  (1867), 
pp.  615-634. 


GEOGNOSY  311 

chlorite,  and  sericite.  Occasionally  these  pliable  scales  are 
so  arranged  as  to  give  a  certain'  flexibility  to  the  stone 
(flexible  sandstone).  This  rock  occurs  in  the  southeastern 
States  of  North  America;  also  in  Brazil,  as  the  matrix  in 
which  diamonds  are  found. 

Siliceous  schist  (Lydian  stone,  Lydite,  Kiesel- 
schiefer)  has  already  been  described  (p.  268)  among  the 
stratified  rocks;  but  ilralso  occurs  among  the  crystalline 
schists,  sometimes  as  the  result  of  the  pulverization  of 
quartzose  rocks  (mylonite). 

Quartzite  (Quartz-rock),  though  not  properly  a  schistose 
rock,  may  be  most  conveniently  considered  here,  as  it  is  so 
constant  an  accompaniment  of  the  schists,  and,  like  them, 
can  often  be  directly  traced  to  the  alteration  of  former  sedi- 
mentary formations.  It  is  a  granular  to  compact  mass  of 
quartz,  generally  white,  sometimes  yellow  or  red  with  a 
characteristic  lustrous  fracture.  It  occurs  in  thin  and  thick 
beds  in  association  with  schists,  sometimes  in  continuous 
masses  several  thousand  feet  thick.  In  Scotland  it  forms 


Fig.  37.— Contorted  Micaceous-schist,  Pig.  38.— Microscopic   Structure  of 

as  seen  under  the  microscope  with  Quartzite.    (Magnified  30  diame- 

a  magnifying  power  of  50  diameters.  ters.) 

ranges  of  mountains,  and  is  there  frequently  accompanied 
by  beds  of  limestone,  which  in  Sutherlandshire  contain 
Cambrian  fossils.816 

Even  to  the  naked  eye,  the  finely  granular  or  arenaceous 
structure  of  qiiartzite  is  distinctly  visible.  Microscopic  ex- 
amination shows  this  structure  still  more  clearly,  and  leaves 
no  doubt  that  the  rock  originally  consisted  of  a  tolerably 
pure  quartz-sand  (Fig.  38).  More  or  less  distinct  evidence 

818  See  the  chapters  on  the  Pre-Cambrian  and  Cambrian  systems  postea. 
On  the  metarnorphic  quartzoae  rocks  of  Morbihan,  France,  see  Barrels,  Ann. 
Soc.  Geol.  Nord,  xi.  (1884). 


312  TEXT-BOOK    OF   GEOLOGY 

of  crashing  and  deformation  of  the  grains  may  often  be  ob- 
served, likewise  proof  of  the  transfusion  of  a  siliceous  cement 
among  the  particles.  This  cement  was  probably  produced 
by  the  solvent  action  of  heated  water  upon  the  quartz  grains, 
which  seem  to  shade  off  into  each  other,  or  into  the  interven- 
ing silica.  It  is  owing,  no  doubt,  to  the  purely  siliceous 
character  of  the  grains  that  the  blending  of  these  with  the 
surrounding  cement  is  so  intimate  as  often  to  give  the  rock 
an  almost  flinty  homogeneous  texture.  That  quartzite,  as 
here  described,  is  an  original  sedimentary  rock,  and  not  a 
chemical  deposit,  is  shown  not  only  by  its  granular  texture, 
but  by  the  exact  resemblance  of  all  its  leading  features  to 
ordinary  sandstone — false-bedding,  alternation  of  coarser  and 
finer  layers,  worm-burrows,  and  fucoid-casts.  The  lustrous 
fracture  that  distinguishes  this  rock  from  sandstone  is  due 
to  the  exceedingly  firm  cohesion  of  the  component  grains, 
which  break  across  rather  than  separate,  and  to  the  conse- 
quent production  of  innumerable  minute  clear  vitreous  sur- 
faces of  quartz.  A  sandstone,  on  the  other  hand,  has  its 
grains  so  loosely  coherent  that  when  the  rock  is  broken  the 
fracture  passes  between  them,  and  the  new  surface  obtained 
presents  innumerable  dull  rounded  grains. 

Besides  occurring  in  alternation  with  schists,  quartzite 
is  also  met  with  locally  as  an  altered  form  of  sandstone, 
which,  when  traversed  by  igneous  dikes,  is  indurated  for  a 
distance  of  a  few  inches  or  feet  from  the  intrusive  mass. 
These  local  productions  of  quartzite  show  the  characteristic 
lustrous  fracture,  and  have  not  yet  been  distinguished  by 
the  microscope  from  the  quartz-rock  of  wide  metamorphic 
regions.  There  is  yet  another  condition  under  which  this 
rock,  or  one  of  analogous  structure,  may  be  seen.  Highly 
silicated  bands,  having  a  lustrous  aspect,  fine  grain,  and 
great  hardness,  occur  among  the  unaltered  shales  and  other 
strata  of  the  Carboniferous  system.  In  such  cases  the  sup- 
position of  any  general  metamorphism  being  inadmissible, 
we  may  infer  either  that  these  quartzose  bands  have  been 
indurated,  for  example,  by  the  passage  through  them  of 
thermal  silicated  water,  or  that  they  are  an  original  for- 
mation. 

Schistose  Conglomerate  Rocks. — In  some  regions  of  schists, 
not  only  bands  of  quartzite  occur,  representing  former  sand- 
stones, but  also  pebbly  or  conglomeratic  bands,  in  which 
pebbles  of  quartz  and  other  materials  from  less  than  an  inch 
to  more  than  a  foot  in  diameter  are  imbedded  in  a  foliated 
matrix,  which  may  be  phyllite,  mica-schist,  gneiss,  quart- 


GEOGNOSY  313 

zite,  etc.317  Examples  of  this  kind  are  found  in  the  pass 
of  the  Tete  Noire  between  Martigny  and  Chamouni,  in  the 
Saxon  granulite  region,  in  the  Bergen  region  of  Norway,  in 
the  northwest  of  France,  in  northwest  Ireland,  in  the  islands 
of  Islay  and  Garvelloch,  and  in  Perthshire  and  other  parts 
of  the  central  Highlands  of  Scotland.  The  pebbles  are  not 
to  be  distinguished  from  the  water- worn  WOCKS  of  ordinary 
conglomerates;  but  the  original  matrix  which  incloses  them 
has  been  so  altered  as  to  acquire  a  micaceous  foliated  struc- 
ture, and  to  wrap  the  pebbles  round  as  with  a  kind  of  glaze. 
These  facts,  like  those  already  referred  to  in  the  structure 
of  quartzite  and  argillaceous  and  quartz-schist,  are  of  con- 
siderable value  in  regard  to  the  theory  of  the  origin  of  some 
crystalline  schists. 

3.  PYROXENE-ROCKS.— Augite-schist — a  fine-grained  schis- 
tose aggregate  of  pale  or  dark-green  augite,  with  sometimes 
quartz,  plagioclase,  magnetite,  or  chlorite;  found  rarely 
among  the  crystalline  schists.  Among  the  schistose  rocks 
of  the  Taunus,  Lessen  has  described  some  interesting  varie- 
ties under  the  name  of  Augite-schist  (Augitschiefer).  They 
are  green,  compact,  sometimes  soft  and  yielding  to  the 
finger-nail,  usually  distinctly  schistose,  and  interbedded  with 
the  gneisses  and  schists.  They  are  composed  of  a  fine  dull 
diabase-like  ground-mass,  through  which  are  dispersed  crys- 
tals of  augite,  1  to  2  mm.  in  length,  which  in  the  typical 
varieties  are  the  only  components  distinctly  recognizable  by 
the  naked  eye."18  Aogite-rock — a  granular  aggregate  of  augite 
(with  tourmaline,  sphene,  scapolite,  etc.),  found  in  beds  in 
the  Laurentian  limestone  of  Canada.  Malacolite-rock 
is  a  pale  granular  to  compact,  or  even  fibrous  aggregt^te  of 
malacolite  found  in  beds  in  crystalline  limestone  (Riesenge- 
birge).  Schistose  Cabbro — a  granular  to  schistose  aggregate  of 
plagioclase  and  diallage,  occurs  in  lenticular  bands  among 
the  amphibolites  and  granulites  of  the  crystalline  schists. 
The  diailage  may  occur  in  conspicuous  crystals,  and  is  some- 
times associated  with  abundant  olivine,  as  in  ordinary  gab- 
bro  (p.  268). 2" 


817  Prof.  "Wichmann  describes  some  curious  examples  of  serpentine  conglom- 
erates.    See  his  paper  in  "Beitrage  zur  Geologie  Ost-Asiens  und  Australiens, " 
ii.  pp.  35,  111.     On  the  conglomerate- schists  of  Saxony,  see  A.  Sauer,  "GeoL 
Specialkarte  Sachsen,"  Sect.  "Elterlein,"  also  Lehmann'a  "Altkryst.  Schiefer- 
gesteine,"  p.  124.     Reusch,  "Silurfossiler  og  Pressede  Konglomerater, "  Chris- 
tiania,  1882.     Barrois,  Ann.  Soc.  Geol.  Nord.  xi.  1884. 

818  Lessen,  Zeitsch.  Deutscu.  Geol.  Ges.  xix.  (1867),  p.  598. 

219  Rocks  of  this  character  occur  in  the  Saxon  "Granulitgebirge"  and  also 
GEOLOGY— Yol.  XXIX— 14 


314  TEXT-BOOK   OF   GEOLOGY 

These  pyrpxenic  intercalations  among  the  schists,  like 
the  hornblendic  and  olivine  bands  mentioned  below,  seem  to 
represent  bands  of  igneous  material  (lavas  or  tuffs)  either 
erupted  contemporaneously  with  the  deposition  of  the  origi- 
nal material  of  the  schists,  or  subsequently  intruded  into  it, 
and  thereafter  exposed  to  the  metamorphism  which  produced 
the  foliation  of  the  schists. 

4.  HORNBLENDE-ROCKS. — Amphibolites — a  name  applied  to 
a  group  of  rocks,  composed  mainly  of  hornblende,  some- 
times schistose,  sometimes  thick-bedded.  Besides  the  horn- 
blende, numerous  other  minerals,  such  as  are  common  among 
the  schists,  likewise  occur — orthoclase,  plagioclase,  quartz, 
augite  and  varieties,  garnet,  zoisite,  mica,  rutile,  etc. 
Where  the  rock  is  schistose,  it  becomes  an  amphibolite-schist 
or  hornblende-schist:  or  if  the  hornblende  takes  the  form  of  ac- 
tinolite,  Actinolite-schist.  Glaucophane-schist 
— a  bluish-gray  or  black  rock,  in  which  the  hornblende  oc- 
curs in  the  form  of  glaucophane,  forms  large  masses  in  the 
Southern  Alps,  and  occurs  locally  in  Anglesey.  Where  an 
amphibolite  is  not  schistose,  it  used  to  be  termed  hornblende- 
rock.  Nephrite  (Jade)  is  a  compact,  extremely  finely 
fibrous  variety.  The  presence  of  other  minerals  in  notice- 
able quantity  may  furnish  names  for  other  varieties.  Thus, 
where  plagioclase  (and  some  orthoclase)  occurs,  the  rock  be- 
comes a  F  elsp  ar-am  p  h  i  bo  1  i  t  e,  Dioritic  amphib- 
olite, or  Di  o  rite-schist.820  Amphibolites  occur  as 
bands  associated  with  gneiss  and  other  schistose  formations. 
It  was  suggested  by  Jukes  that  they  may  possibly  represent 
former  beds  of  hornblendic  or  augitic  lava  and  tuff,  which 
have  been  metamorphosed  together  with  the  strata  among 
which  they  were  intercalated.  This  suggestion  has  received 
confirmation  from  the  researches  of  the  Geological  Survey 
in  the  north  of  Scotland  and  in  Ireland,  where  what  were 
doubtless  originally  pyroxenic  masses  erupted  prior  to  the 
metamorphism  of  the  region,  have  had  their  augite  changed 
by  paramorphism  into  hornblende,  and  have  partially  as- 
sumed a  foliated  structure,  passing  into  Epidiorite,  Epi- 
d  i  o  r  i  t  e-s  c  h  i  s  t,  amphibolite-schists,  and  even  serpentine. 


in  Lower  Austria.  F.  Becke,  Tschermak's  Min.  Mitth.  IV.  p.  352.  J.  Leh- 
mann's  "Untersuchungen  iiber  die  Entstehung  der  Altkrystallinischen  Schiefer- 
gesteine,"  Bonn,  1884,  p.  190.  On  the  diabase-schists  of  the  Taunus,  see  L. 
Milch,  Zeitsch.  Deutsch.  Geol.  Ges.  xli.  (1889),  p.  394. 

**°  See  F.  Becke,  Tschermak's  Min.  Mitth.  IV.  p.  233.  The  author  likewise 
distinguishes  diallage-amphibolite,  garaet-amphibolite,  salite-amphibolite,  zoisite- 
amphibolite. 


GEOGNOSY  315 

The  connection  of  some  schists  with  original  masses  of  dio- 
rite,  gabbro,  and  diabase  has  been  pointed  out  by  Lehmann 
and  subsequently  by  many  other  observers."1 

5.  G-ARNET-Rpcks.—  Eclogite,   one  of  the  most  beautiful 
members  of  the  crystalline-schist  series,  is  a  granular  aggre- 
gate of  grass-green  omphacite  (pyroxene)  and  red  garnet, 
through  which  are  frequently  dispersed  bluish  kyanite  and 
white  mica.     It  occurs  in  bands  in  the  Archa3an  gneiss  and 
mica-schist.     To  those  varieties  where  the  kyanite  becomes 
predominant,  the  name  of  K  y  a  n  i  t  e-r  o  c  k  has  been  given. 
Garnet-rock   is    a   crystalline-granular    rock    composed 
mainly  of  garnet,  with  hornblende  and  magnetite;    by  the 
diminution   of   the   garnet   it   passes   into   an   amphibolite. 
Kinzigite — a    crystalline    schistose    rock,    composed    of 
plagioclase,   garnet,   and   black   mica,   found   in   the  Black 
Forest  (Kinsig)  and  the  Odenwald. 

6.  EPIDOTE-ROCKS. — Epidosite  (Pistacite-rock) — an  aggre- 
gate of  bright  green  epidote  with  some  quartz,  occurs  with 
chlorite-schist  (Canada),  with  granite  and  serpentine  (Elba), 
and  with  syenite.     E  p  i  d  o  t  e-s  c  h  i  s  t,  a  schistose  greenish 
rock,  with  silvery  lustre  on  the  foliation  surfaces,  composed 
of  epidote,  sericite,  magnetite,  quartz,  calcite,  plagioclase, 
and  specular  iron.4" 

7.  CHLORITE-ROCKS. — Chlorite-schist — a  scaly  schistose  ag- 
gregate of  greenish  chlorite,  usually  with  quartz  and  often 
with  felspar,  talc,  mica,  or  magnetite,  the  last-named  min- 
eral frequently  appearing  in  beautifully  perfect  disseminated 
octohedra.     Occurs  with  gneiss  and  other  schists  in  evenly 
bedded  masses. 

8.  TALC-ROCKS. — Talc-schist — a  schistose  aggregate  of  scaly 
talc,  often  with  quartz,  felspar,  and  other  minerals;  having 
an  unctuous  feel,  and  white  or  greenish  color.    Occurs  some- 
what rarely  in  beds  associated  with  mica-schist  and  clay- 
slate,    and    frequently  contains   magnetite,    chlorite,    mica, 
kyanite,    and    other    minerals,    including    carbonates.      A 
massive  variety,  composed  of  a  finely  felted  aggregate  of 


221  "Untersuchungen  iiber  die  Entstehung  der  Altkrystall  Schief. "  See  also 
Giimbel,  "Die  Palaolitischen  Eruptivgesteine  des  Fichtelgebirges, "  Munich, 
1874,  p.  9;  Teall,  Quart.  Journ.  Geol.  Soc.  xli.  (1883),  p.  133;  "British  Petro- 
graphy," p.  198.  Hatch,  Mem.  Geol.  Survey,  Explanation  of  Sheets,  138,  139, 
Ireland,  p.  49.  Hyland,  Mem.  Geol.  Survey,  Explanations  of  Northwest  Done- 
gal, and  of  Southwest  Donegal,  Petrographical  appendices,  also  postea,  Book 
IV.  pt.  viii.  G.  H.  Williams,  Bull.  U.  S.  Geol.  Surv.  No.  62,  1890. 

422  See  Wichmann  on  Rocks  of  Timor,  "Beitrage  zur  Geologie  Ost-Asiens 
und  Australiens,"  II.  part  2,  p.  97,  Leyden,  1884. 


316  TEXT-BOOK    OF   GEOLOGY 

scales  of  talc,  with  chlorite  and  serpentine,  is  called 
Potstone  (Topfstein).  Many  rocks  with  a  soapy  or 
unctuous  feel  have  been  classed  as  talc-schist,  which. con- 
tain no  talc,  but  a  variety  of  mica  (sericite-schist,  etc.). 
Talc-schist,  though  not  specially  abundant,  occurs  in  con- 
siderable mass  in  the  Alps  (Mont  Blanc,  Monte  Rosa, 
Carinthia,  etc.),  and  is  found  also  among  the  Apennine 
and  Ural  mountains. 

9.  OLIVINE-ROCKS,   or  PERIDOTITES  of  the  Crystalline 
Schists.""     Eocks  of  which  olivine  forms  a  main  constit- 
uent, occur  as  subordinate  bands  or  irregular  masses  asso- 
ciated with  gneisses  and  other  schistose  rocks.     They  were 
probably  eruptive  masses,  contemporaneous  with  or  subse- 
quent to  the  surrounding  gneisses  and  schists  (p.  314).     The 
olivine  is  commonly  associated  with  some  pyroxenic  min- 
eral, hornblende,  garnet,  etc.     Some  of  the  rocks  mentioned 
on  p.  300  may  also  be  included  here.     Dunite,  for  example, 
which  occurs  in  apparently  eruptive  form  at  Dun  Mountain, 
near  Nelson,  New  Zealand,  is  found  in  North  Carolina  in 
beds  with   laminated  structure  intercalated  in  hornblende- 
gneiss.     Many  of  these  rocks  have  undergone  much  crush- 
ing   and    deformation,    and    pass    into    foliated    forms    of 
Serpentine,  which  must  thus  be  reckoned  as  one  of  the 
schistose  as  well  as  one  of  the  eruptive  series.     Some  re- 
markable  schistose   serpentines   occur    interbedded    among 
phyllites,  mica-schists,  and  limestones  in  Banffshire. 

10.  FELSITOID-EOCKS. — These  are  distinguished   by  an 
exceedingly   compact   felsite-like   matrix.      They    occur  in 
beds  or  bed-like  masses,  sometimes  in  districts  of  contact 
metamorphism,   sometimes  associated  with  vast  masses  of 
schists. 

Halleflinta — an  exceedingly  compact,  hornstone-like,  fel- 
sitic,  gray,  yellowish,  greenish,  reddish,  brownish,  or  black, 
rock,  composed  of  an  intimate  mixture  of  microscopic  par- 
ticles of  felspar  and  quartz,  with  fine  scales  of  mica  and 
chlorite.  It  breaks  with  a  splintery  or  conchoidal  fracture, 
presents  under  the  microscope  a  finely-crystalline  structure, 
occasionally  with  nests  of  quartz,  and  is  only  fusible  in  fine 
splinters  before  the  blow-pipe.  Some  of  the  rocks  to  which 
this  name  has  been  applied  are  probably  felsitic  lavas; 
others,  though  externally  presenting  a  resemblance  to  fel- 

823  See  Tschermak,  Sitzb.  Akad.  Wissen.,  Vienna,  Ivi.  (1867).  F.  Becke, 
Tschermak's  Min.  Mitth.  IV.  (1882),  p.  322.  E.  Dathe,  Neues  Jahrb.  1876, 
pp.  265-337. 


GEOGNOSY  317 

site,  occur  in  beds  intimately  associated  with  foliated  rocks 
(Norway),  and  may  be  metamorphic  products  (perhaps  al- 
tered fine  sediments)  due  to  the  same  series  of  changes  that 
gave  rise  to  the  crystalline  schists  among  which  they  lie.2"* 

Adinole  (Adinole-schist) — a  rock  externally  resembling 
the  last,  but  distinguished  from  it  by  its  greater  fusibility. 
It  is  an  intimate  mixture  of  quartz  and  albite,  containing 
about  ten  per  cent  of  soda.  It  is  a  product  of  alteration, 
being  found  among  the  altered  Carboniferous  shales  around 
the  eruptive  diabases  of  the  Harz,  in  the  altered  Devonian 
rocks  of  the  Taunus,  and  in  the  altered  Cambrian  rocks  of 
South  Wales."6 

Porphytoid — a  name  bestowed  upon  certain  rocks  composed 
of  a  felsite-like  ground-mass  which  has  assumed  a  more  or 
less  schistose  structure  from  the  development  of  micaceous 
scales,  and  which  contains  porphyritically  scattered  crystals 
of  felspar  and  quartz.  The  felspar  is  either  orthoclase  or  al- 
bite, and  may  be  obtained  in  tolerably  perfect  crystals.  The 
quartz  occasionally  presents  doubly  terminated  pyramids. 
The  micaceous  mineral  may  be  paragonite  or  sericite.  Por- 
phyroid occurs  in  circumstances  which  suggest  considerable 
mechanical  deformation,  as  among  the  schistose  rocks  of 
Saxon v,"8  in  the  Paleozoic  area  of  the  Ardennes,"7  as- well 
as  in  Westphalia  and  other  parts  of  Europe.**8  Some  por- 
pbyroids  are  probably  sheared  forms  of  quartz-porphyry, 
felsite,  or  some  similar  rock;  others  may  be  more  of  the 
nature  of  tuffs. 

11.  QUARTZ-  AND  TOUBMALINE-ROCKS.—  Tourmaline-schist 
(Schorl -schist,    schorl-rock),    a    blackish,    finely    granular, 
quartzose   rock    with    abundant    granules   and    needles   of 
black    tourmaline   (schorl),    which    occurs    as   one   of    the 
products  of  contact-metamorphism  in  the  neighborhood  of 
some  granites  (Cornwall). 

12.  QUARTZ-  AND   MICA-BOCKS. — Mica-schist  (Mica-slate, 
Glimmerschiefer),  a  schistose  aggregate  of  quartz  and  mica, 


224  For  analyses  see  H.  Santesson,  "Kemiska  Bergsartanalyser,"  8vo,  Stock- 
holm, 1877. 

226  Lessen,  Zeitsch.  Deutsch.  Geol.  Gesel.  xix.  (1867),  p.  573.  See  also 
Quart.  Journ.  Geol.  Soc.  xxxix.  (1883),  pp,  302,  320.  Rosenbusch,  "Mikro- 
skopische  Physiographic, "  ii.  p.  235.  F.  Posepny,  Tschermak's  Mineral.  Mitth. 
x.  175. 

288  Rothpletz,  Geol.  Survey  Saxony,  Explanation  of  Section  Rochlitz. 

221  De  la  Vallee  Poussin  and  Renard,  Mem.  Couronnees  Acad.  Roy.  Belg. 
1876,  p.  85. 

228  Lessen,  Site.  Gesellsch.  Naturf.  Freunde,  1883,  No.  9. 


318  TEXT-BOOK   OF   GEOLOGY 

the  relative  proportions  of  the  two  minerals  varying  widely 
even  in  the  same  mass  of  rock.  Each  is  arranged  in  lentic- 
ular wavy  laminae.  The  quartz  shows  great  inconstancy 
in  the  n amber  and  thickness  of  its  folia.  It  often  presents 
a  granular  character,  like  that  of  quartz-rock,  or  passing  into 
granulite.  The  mica  lies  in  thin  plates,  sometimes  so  dove- 
tailed into  each  other  as  to  form  long  continuous  irregular 
crumpled  folia,  separating  the  quartz  layers,  and  often  in 
the  form  of  thin  spangles  and  membranes  running  in  the 
C[uartz.  (Figs.  36  and  37.)  As  the  rock  splits  open  along 
its  micaceous  folia,  the  quartz  is  not  readily  seen  save  in- 
a  cross  fracture. 

The  mica  in  typical  mica-schist  is  generally  a  white 
variety;  but  it  is  sometimes  replaced  by  a  dark  species. 
In  many  lustrous,  unctuous  schists  which  are  now  found 
to  have  a  wide  extent,  the  silvery  foliated  mineral  is  ascer- 
tained to  be  a  mica  (margarodite,  damourite,  etc.),  and  not 
talc,  as  was  once  supposed.  These  were  named  by  Dana 
hydro-mica-schists.  Among  the  accessory  minerals,  garnet 
(specially  characteristic),  schorl,  felspar,  hornblende,  kya- 
nite,  staurolite,  chlorite,  and  talc  may  be  mentioned.  Mica- 
schist  readily  passes  into  other  members  of  the  schistose 
family.  By  addition  of  felspar,  it  merges  into  gneiss.  By 
loss  of  quartz  and  increase  01  chlorite,  it  passes  into  chlorite- 
schist,  and  by  loss  of  mica,  into  quartz-schist  and  quartzite. 
By  failure  01  quartz  and  diminution  of  mica,  with  an  in- 
creasing admixture  of  calcite,  it  may  shade  into  calc-mica- 
schist  (see  below),  and  even  into  marble.  Mica-schist  varies 
in  color  mainly  according  to  the  hue  of  its  mica. 

Mr.  Sorby  has  stated  that  thin  slices  of  some  mica-schists, 
when  examined  under  the  microscope,  show  traces  of  orig- 
inal grains  of  quartz-sand  and  other  sedimentary  particles 
of  which  the  rock  at  first  consisted.  He  has  ajso  found 
indications  of  what  he  supposes  to  have  been  current- 
bedding  or  ripple-drift,  like  that  seen  in  many  fine  sedi- 
mentary deposits,  and  he  concludes  that  mica-schist  is  a 
crystalline  metamorphosed  sedimentary  rock.229  In  many, 
if  not  in  most  cases,  however,  the  foKation  does  not  corre- 
spond with  original  bedding,  but  with  structural  planes 
(cleavage,  faulting)  superinduced  by  pressure,  tension,  or 

229  Q.  J.  Geol.  Soc.  (1863),  p.  401,  and  his  address  in  vol.  xxxvi.  (1880),  p.  85. 
The  apparent  current-bedding  of  many  granulitic  and  other  metarnorphic  rocks 
is  certainly  deceptive,  and  must  be  due  to  planes  of  shearing  or  slipping  in  the 
mechanical  movements  which  produced  the  metamorphism. 


GEOGNOSY  319 

otherwise,  upon  rocks  which  may  not  always  have  been 
of  sedimentary  origin. 

Among  the  varieties  of  mica-schist  may  be  mentioned 
S  e  r  i  c  i  t  e-s  c  h  i  s  t  (which  may  be  also  included  among  the 
phyllites),  composed  of  an  aggregate  of  fine  folia  of  the 
silky  variety  of  mica  called  sericite,  in  a  compact  honestone- 
like  quartz;  Paragonite-schist,  where  the  mica  is  the 
hydrous  soda  variety,  paragonite;  Gr  n  eiss-mica-sc  hi  st, 
containing  dispersed  kernels  of  orthoclase.  Some  of  these 
rocks  contain  little  or  no  quartz,  the  place  of  which  is 
taken  by  felspar.  Gal  c-m  i  c  a-s  c  h  i  s  t,  a  schistose  calca- 
reous rock,  which  in  many,  if  not  in  all  cases,  was  origi- 
nally a  limestone  with  more  or  less  muddy  impurity.  The 
carbonate  of  lime  has  assumed  a  granular-crystalline  form, 
while  the  aluminous  silicates  have  recrystallized  as  fine 
scales  of  white  mica.  *  Tremolite,  zoisite,  and  other  min- 
erals are  not  infrequent  in  this  rock. 

Normal  mica-schist,  together  with  other  schistose  rocks, 
forms  extensive  regions  in  Norway,  Scotland,  the  Alps, 
and  other  parts  of  Europe,  and  vast  tracts  of  the  "  Archa3an" 
regions  of  North  America.  Some  of  its  varieties  are  also 
found  encircling  granite  masses  (Scotland.  Ireland,  etc.)  as 
a  zone  or  aureole  of  contact-metamorphism  from  a  few  yards 
to  a  mile  or  so  broad,  which  shades  away  into  unaltered 
graywacke  or  slate  outside.  In  these  cases,  mica-schist  is 
unquestionably  a  metamorphosed  condition  of  ordinary 
sedimentary  strata,  the  change  being  connected  with  the 
extravasation  of  granite.  (Book  IV.  Part  VIII.) 

Though  the  possession  of  a  fissile  structure,  showing 
abundant  divisional  surfaces  covered  with  glistening  mica, 
is  characteristic  of  mica-schist,  we  must  distinguish  between 
this  structure  and  that  of  many  micaceous  sandstones  which 
can  be  split  into  thin  seams,  each  splendent  with  the  sheen 
of  its  mica-flakes.  A  little  examination  will  show  that  in 
the  latter  case  the  mica  has  not  crystallized  in  situ,  but 
exists  merely  in  the  form  of  detached  worn  scales,  which, 
though  lying  on  the  same  general  plane,  are  not  welded 
into  each  other  as  in  a  schist;  also  that  the  quartz  does  not 
exist  in  folia  but  in  rounded  separate  grains. 

13.  QUARTZ-  AND  FELSPAR-KOCKS. — The  replacement  of 
the  mica  of  a  mica-schist  by  felspar,  or  the  disappearance 
of  the  mica  from  a  gneiss,  gives  rise  to  an  aggregate  of  fel- 
spar and  quartz.  Such  a  rock  may  be  observed  in  thin 
bands  or  courses,  alternating  with  the  surrounding  mass. 
In  mineral  composition,  it  may  be  compared  to  the  quartz- 


320  TEXT-BOOK   OF   GEOLOGY 

porphyries  or  granite-porphyries  of  the  massive  rocks,  but 
it  is  usually  distinguishable  by  a  more  or  less  foliated  struc- 
ture, and  by  the  absence  of  felsitic  ground-mass. 

14.  QUARTZ-,  FELSPAR-,  AND  MICA-ROCKS. — Gneiss. — 
This  name,  formerly  restricted  to  a  schistose  aggregate  of 
orthoclase  (sometimes  micro  line  or  a  plagioclastic  felspar, 
either  separate  or  crystallized  together),  quartz,  and  mica, 
is  now  commonly  employed  in  a  wider  sense  to  denote  the 
coarser  schists  which  so  often  present  granitoid  characters."* 
Many  gneisses,  indeed,  differ  from  granite  chiefly  in  the 
foliated  arrangement  of  the  minerals.  The  quartz  some- 
times contains  abundant  liquid  inclusions,  in  which  liquid 
carbon-dioxide  has  been  detected.  The  relative  proportions 
of  the  minerals,  and  the  manner  in  which  they  are  grouped 
with  each  other,  present  great  variations.  As  a  rule,  the 
folia  are  coarser,  and  the  schistose  character  less  perfect 
than  in  mica-schist.  Sometimes  the  quartz  lies  in  tolerably 
pure  bands,  a  foot  or  even  more  in  thickness,  with  plates 
of  mica  scattered  through  it.  These  quartz  layers  may  be 
replaced  by  a  crystalline  mixture  of  quartz  and  felspar, 
or  the  felspar  will  take  the  form  of  independent  lenticular 
folia,  while  the  laminae  of  mica  which  lie  so  abundantly  in 
the  rock  give  it  its  fissile  structure.  The  felspar  of  many 
gneisses  presents  under  the  microscope  a  remarkable  fibrous 
structure,  due  to  the  crystallization  of  fine  lamella  of  some 
plagioclase  (albite  or  oligoclase)  in  the  main  mass  of  ortho- 
clase or  microcline.a81  Among  the  accessory  minerals,  gar- 
net, tourmaline  or  schorl,  hornblende,  apatite,  graphite,  py- 
rites, and  magnetite  may  be  enumerated. 

There  can  be  no  doubt  that  many  gneisses  owe  their 
characteristic  schistose  structure  to  the  crushing  and  shear- 
ing of  some  original  eruptive  rock  such  as  granite.  In- 
stances, however,  occur  where  the  materials  are  segregated 
in  bands  which  so  closely  resemble  those  of  true  flow-struc- 
ture or  segregation  in  igneous  bosses  and  sheets  as  to  suggest 
that  they  may  possibly  have  resulted  from  the  movement 
of  a  still  unconsolidated  eruptive  mass  (pp.  306,  1022). 
Analogies  to  such  structures  may  be  observed  among 
ancient  and  modern  lavas. 

830  See  Kalkowsky's  "Gneissformation  des  Eulengebirges, "  Leipzig,  1878; 
Lehmann's  "Altkrystallinische  Schief ergesteine, "  1884;  F.  Becke,  Tschermak'g 
Min.  Mitth.  1882,  p.  194;  E.  "Weber,  op.  cit.  1884,  p.  1,  and  postea  Book  IV. 
Part  VIII.  §  ii.  and  Book  VI.  Pre-Cambrian. 

»»  P.  Becke  (Tscharmak's  Min.  Mitth.  1882,  (iv.)  p.  1»8)  described  thto 
structure  and  named  it  micropertMte. 


GEOGNOSY  321 

Many  varieties  of  gneiss  occur.  Some  are  distinguished 
by  peculiarities  of  structure  or  composition,  as  Granite- 
gneiss,  where  the  schistose  arrangement  is  so  coarse  as 
to  be  unrecognizable,  save  in  a  large  mass  of  the  rock; 
Diorite-gneiss,  gabbr o-g n e i s s,  composed  of  the 
materials  of  a  diorite  or  gabbro  but  with  a  coarsely  schis- 
tose structure;  Porphyritic  gneiss  or  Augen- 
gneiss,  in  which  large  eye-like  kernels  of  orthoclase  or 
quartz  are  dispersed  through  a  finer  matrix  and  represent 
larger  crystals  or  crystalline  aggregates  which  have  been 
broken  down  and  dragged  along  by  shearing  movements  in 
the  rock.  Other  varieties  are  named  from  the  occurrence 
in  them  of  one  or  more  distinguishing  minerals,  as  Horn- 
blende-gneiss (syenitic  gneiss),  in  which  hornblende 
occurs  instead  of  or  in  addition  to  mica;  Protogine- 
gneiss,  where  the  ordinary  mica  is  altered  into  chlorite  or 
a  talc-like  substance;  Serici  te-g  ne  iss,  a  schistose  ag- 
gregate of  sericite,  albite,  (juartz,  with  less  frequently  white 
and  black  mica  and  a  chloritic  mineral  ;23a  A  u  g  1 1  e-g  n  e  i  s  s, 
containing  an  augitic  mineral  (not  of  the  diallage  group)  and 
potash-felspar  or  potash-soda-felspar  or  scapolite,  with  horn- 
blende (which  has  often  crystallized  parallel  with  the  augite), 
brown  mica,  more  or  less  quartz,  and  also  frequently  with 
garnet,  calcite,  titanite,  etc.;"3  P  1  ag  i  oclas  e-g  n  ei  ss, 
with  plagioclase  more  abundant  than  orthoclase,  sometimes 
containing  hornblende,  sometimes  augite;  Cordierite- 
gneiss,  with  the  bluish  vitreous  mineral  cordierite. 

The  most  typical  gneisses  occur  among  the  so-called 
"Archaean  rocks,"  of  which  they  form  the  leading  type, 
and  where  they  probably  represent  original  eruptive  rocks. 
(See  Book  VI.'  Part  L)  They  cover  considerable  areas  in 
Scandinavia,  N.  W.  Scotland,  Bohemia,  Bavaria,  Erzge- 
birge,  Moravia,  Central  Alps,  Canada,  etc.  But  rocks  to 
which  the  name  of  gneiss  cannot  be  refused  appear  also 
among  the  products  of  the  metamorpliism  of  various  strati- 
fied formations.  Such  are  the  gneisses  associated  with  iriany 
other  crystalline  schists  among  the  altered  Cambrian  and 
Silurian  rocks  of  Scotland,  Norway,  and  New  England, 

m  K.  A.  Lessen,  Zeitsch.  Deutsch.  Geol.  Ges.  xix.  (1867),  p.  565. 

838  The  occurrence  of  augite  as  an  abundant  constituent  of  some  gneisses 
has  been  made  known  by  microscopic  research.  Rocks  of  this  naiure  occur  in 
Sweden  (A.  Stelzner,  N.  .Tahrb.  1880  (ii.),  p.  103),  and  have  been  fully  de- 
scribed from  Lower  Austria  (P.  Becke,  Tschermak's  Min.  Mitth.  1882  (iv.),  pp. 
219-365).  They  are  likewise  well  developed  among  the  oldest  gneisses  of  the 
northwest  of  Sutherland  in  Scotland. 


322  TEXT-BOOK   OF  GEOLOGY 

the  altered  Devonian  rocks  of  the  Taunus,  and  other  re- 
gions, which  will  be  described  in  Book  IV.  Part  VIII. 
Some  of  these  may  also  be  eruptive  granites,  dioritest 
etc.,  which  have  undergone  shearing  and  have  acquired 
a  schistose  character. 

15.  QUARTZ-,  FELSPAR-,  AND  GARNET-ROCKS.— Cranolite"* 
(Eurite-schistoide,  Leptynite  of  French  authors,  Weiss-stein) 
— a  fine-grained  granular  aggregate  of  pale  reddish,  yellow- 
ish, or  white  felspar  with  quartz  and  small  red  garnets,  oc- 
casionally with  kyanite,  biotite,  and  microscopic  rutile  and 
tourmaline.     The  felspar,  which  is  the  predominant  constit- 
uent, presents  the  peculiar  fibrous  structure  referred  to  in 
the  foregoing  description  of  gneiss  (microperthite,   micro- 
cline),    and   appears    seldom   to    be    true   orthoclase.     The 
quartz  is  conspicuous  in  thin  partings  between  thicker  more 
lelspathic  bands,  giving  a  distinctly  fissile  bedded  character 
to  the  mass.     A  dark  variety,  interstratified  with  the  normal 
rock,  is  distinguished  by  the  presence  of  microscopic  augite 
or  diallage  (Augitgranulite  of  Saxony).     Granulite  occurs  in 
bands  among  the  gneiss  and  other  members  of  the  crystal- 
line schist  series  in  Saxony,  Bohemia,  Lower  Austria,  the 
Vosges,  and  Central  France.     The  term  "granulite"  is  also 
employed  in  a  structural  sense  to  denote  a  rock  which  has 
been  crushed  down  by  dynamic  metamorphism,  and  has  ac- 
quired this  characteristic  fine  granular  structure.     (See  pp. 
177,  211.) 

16.  FELSPAR-  AND  MICA-ROCKS. — Rocks  composed  essen- 
tially of  a  schistose  aggregate  of  minutely  scaly  mica  with 
some  felspar,  quartz,  andalusite,  or  other  mineral,  occur  in 
regions  of  metamorphism.     Cornubianite    was  a  name 
proposed  bv  Boase  for  a  rock  composed  of  a  felspar  base, 
witn  abundant  mica.8"     It  is  found  around  the  granite  of 
Cornwall,  of  which  it  is  a  metamorphic  product.     By  some 
writers  this  rock  has  been  associated  with  the  gneisses,  but 
it  is  distinguished  by  the  scarcity  or  absence  of  quartz. 

884  Michel-LeVy  has  proposed  to  reserve  the  names  "Leptynite"  for  schistose 
and  "Granulite"  for  eruptive  rocks.  Bull.  Soc.  Geol.  France,  3d  ser.  ii.  pp. 
177,  189,  iii.  p.  287,  iv.  p.  730,  vii.  p.  760;  Lory,  op.  cit.  viii.  p.  14.  Scheerer, 
Neues  Jahrb.  1873,  p.  673.  Dathe,  N.  Jahrb.  1876,  p.  225;  Z.  Deutsch.  GeoL 
Ges.  1877,  p.  274.  Details  regarding  the  great  development  of  the  granulite  of 
Saxony  (Granulitgebirge)  will  be  found  in  the  explanatory  pamphlets  published 
with  the  sheets  of  the  Geological  Survey  of  Saxony,  especially  those  of  sections 
Rochlitz,  Geringswalde  and  Waldheim.  The  history  of  the  origin  of  granulite 
is  discussed  by  J.  Lehmann,  "Untersuchungen  iiber  die  Entstehung  der  Altkry- 
Btall.  Schiefergesteine." 

236  "Geology  of  Cornwall"  (1832),  pp.  226,  230. 


GEOGNOSY 


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TEXT-BOOK   OF  QEOLOGY 


BOOK   HI 

DYNAMICAL   GEOLOGY 

DYNAMICAL  GEOLOGY  investigates  the  processes 
of  change  at  present  in  progress  upon  the  earth, 
whereby  modifications  are  made  on  the  structure 
and  composition  of  the  crust,  on  the  relations  between  the 
interior  and  the  surface,  as  shown  by  volcanoes,  earth- 
quakes, and  other  terrestrial  disturbances,  on  the  distri- 
bution of  land  and  sea,  on  the  outlines  of  the  land,  on  the 
form  and  depth  of  the  sea -bottom,  on  marine  currents,  and 
on  climate.  Bringing  before  us,  in  short,  the  whole  range 
of  geological  activities,  it  leads  us  to  precise  notions  regard- 
ing their  relations  to  each  other,  and  the  results  which  they 
achieve.  A  knowledge  of  this  branch  of  the  subject  is  thus 
the  essential  groundwork  of  a  true  and  fruitful  acquaintance 
with  the  principles  of  geology.  The  study  of  the  present 
order  of  nature  provides  a  key  for  the  interpretation  of  the 
past. 

The  operations  considered  by  Dynamical  Geology  may 
be  regarded  as  a  vast  cycle  of  change,  into  the  investigation 
of  which  the  student  may  break  at  any  point,  and  round 
which  he  may  travel,  only  to  find  himself  brought  back  to 
his  starting-point.  It  is  a  matter  of  comparatively  small  mo- 
ment at  what  part  of  the  cycle  the  inquiry  is  begun.  The 
changes  seen  in  action  will  always  be  found  to  have  resulted 
from  some  that  preceded,  and  to  give  place  to  others  that 
follow  them. 


DYNAMICAL    GEOLOGY  325 

At  an  early  time  in  the  earth's  history,  anterior  to  any 
of  the  periods  of  which  a  record  remains  in  the  visible  rocks, 
the  chief  sources  of  geological  energy  probably  lay  within 
the  earth  itself.  The  planet  still  retained  much  of  its  initial 
heat,  and  in  all  likelihood  was  the  theatre  of  great  chemical 
changes.  As  it  cooled,  and  as  the  superficial  disturbances 
due  to  internal  heat  and  chemical  action  became  less  marked, 
the  influence  of  the  sun,  which  must  always  have  operated, 
and  which  in  early  geological  times  may  have  been  more 
effective  than  it  afterward  became,  would  then  stand  out 
more  clearly,  giving  rise  to  that  wide  circle  of  surface 
changes  wherein  variations  of  temperature  and  the  circula- 
tion of  air  and  water  ovej*  the  surface  of  the  earth  come  into 
play. 

In  the  pursuit  of  his  inquiries  into  the  past  history  and 
into  the  present  economy  of  the  earth,  the  student  must 
needs  keep  his  mind  ever  open  to  the  reception  of  evidence 
for  kinds,  and  especially  for  degrees,  of  action  which  he  had 
not  before  encountered.  Human  experience  has  been  too 
short  to  allow  him  to  assume  that  all  the  causes  and  modes 
of  geological  change  have  been  definitely  ascertained.  Be- 
sides the  fact  that  both  terrestrial  and  solar  energy  were 
once  probably  more  intense  than  now,  there  may  remain  for 
future  discovery  evidence  of  former  operations  by  heat, 
magnetism,  chemical  change,  or  other  agency,  that  may 
explain  phenomena  with  which  geology  has  to  deal.  "Of 
the  influences,  so  many  and  profound,  which  the  sun  exerts 
upon  our  planet,  we  can  as  yet  only  perceive  a  little.  Nor 
can  we  tell  what  other  cosmical  influences  may  have  lent 
their  aid  in  the  revolutions  of  geology. 

In  the  present  state  of  knowledge,  all  the  geological 
energy  upon  and  within  the  earth  must  ultimately  be  traced 


326  TEXT-BOOK    OF   GEOLOGY 

back  to  the  primeval  energy  of  the  parent  nebula,  or  suii. 
There  is,  however,  a  certain  propriety  and  convenience  in 
distinguishing  between  that  part  of  it  which  is  due  to  the 
survival  of  some  of  the  original  energy  of  the  planet,  and 
that  part  which  arises  from  the  present  supply  of  energy  re- 
ceived day  by  day  from  the  sun.  In  the  former  case,  the 
geologist  has  to  deal  with  the  interior  of  the  earth  and  its 
reaction  upon  the  surface;  in  the  latter,  he  is  called  upon  to 
study  the  surface  of  the  earth,  and  to  some  extent  its  re- 
action on  the  interior.  This  distinction  allows  of  a  broad 
treatment  of  the  subject  under  two  divisions: 

I.  Hypogene  or  Plutonic  Action — the  changes 
within  the  earth,  caused   by  original  internal  heat  and  by 
chemical  action. 

II.  Epigene    or    Surface    Action — .the   changes 
produced  on   the  superficial  parts  of  the  earth,  chiefly  by 
the  circulation  of  air  and  water  set  in  motion  by  the  sun's 
heat. 

PART  I.     HYPOGENE  ACTION 

An  Inquiry  into  the  Geological  Changes  in  Progress 
beneath  the  Surface  of  the  Earth 

In  the  discussion  of  this  branch  of  the  subject,  it  is  use- 
ful to  carry  in  the  mind  the  conception  of  a  globe  still  in- 
tensely hot  within,  radiating  heat  into  space,  and  conse- 
quently contracting  in  bulk.  Portions  of  molten  rocks  from 
inside  are  from  time  to  time  poured  out  at  the  surface. 
Sudden  shocks  are  generated,  by  which  earthquakes  are 
propagated  to  and  along  the  surface.  Wide  geographical 
areas  are  upraised  or  depressed.  In  the  midst  of  these  move- 
ments, the  rocks  of  the  crust  are  fractured,  squeezed, 
sheared,  crumpled,  rendered  crystalline,  and  even  fused. 


DYNAMICAL    GEOLOGY  327 

Section  i.    Volcanoes  and  Volcanic  Action1 
§  1.  Volcanic  Products 

The  term  volcanic  action  (volcanism  or  volcanicity)  em- 
braces all  the  phenomena  connected  with  the  expulsion  of 
heated  materials  from  the  interior  of  the  earth  to  the  surface. 
Among  these  phenomena,  some  possess  an  evanescent  char- 
acter, while  others  leave  permanent  proofs  of  their  existence. 
It  is  naturally  to  the  latter  that  the  geologist  gives  chief  at- 
tention, for  it  is  by  their  means  that  he  can  trace  former 
phases  of  volcanic  activity  in  regions  where,  for  many  ages, 
there  have  been  no  volcanic  eruptions.  In  the  operations 
of  existing  volcanoes,  he  can  observe  only  superficial  mani- 
festations of  volcanic  action.  But  examining  the  rocks  of 
the  earth's  crust,  he  discovers  that  amid  the  many  terrestrial 
revolutions  which  geology  reveals,  the  very  roots  of  former 
volcanoes  have  been  laid  bare,  displaying  subterranean 


1  The  student  is  referred  to  the  following  general  works  on  the  phenomena 
of  volcanoes.  Scrope,  "Considerations  on  Volcanoes,"  London,  1825;  "Vol- 
canoes," London,  2d  edit.  1872;  "Extinct  Volcanoes  of  Central  France,"  Lon- 
don, 1858;  "On  Volcanic  Cones  and  Craters,"  Quart.  Jouru.  Geol.  Soc.  1859. 
Daubeny,  "A  Description  of  Active  and  Extinct  Volcanoes,"  2d  edit.,  London, 
1858.  Darwin,  "Geological  Observations  on  Volcanic  Islands,"  2d  edit.,  Lon- 
don, 1876.  A.  von.  Humboldt,  "Deber  den  Bau  und  die  Wirkung  der  Vul- 
kane,"  Berlin,  1824.  L.  von  Buch,  "Deber  die  Natur  der  vuikanischen  Er- 
scheinungen  auf  den  Canarischen  Inseln,"  Poggend.  Annalen  (1827),  ix.  x. ; 
"Ueber  Erhebungskratere  und  Vulkane,"  Poggend.  Annalen  (1836),  xxxvii. 
E.  A.  von  Hoff,  "Geschichte  der  durch  Ueberlieferung  nachgewiesenen  natur- 
lichen  Veranderungen  der  Erdoberflache"  (part  ii.,  "Vulkane  und  Erdbebeh"), 
Gotha,  1824.  C.  W.  C.  Fuchs,  "Die  vuikanischen  Erscheinungen  der  Erde," 
Leipzig,  1865.  R.  Mallet,  "On  Volcanic  Energy,"  Phil.  Trans.  1873.  J. 
Schmidt,  "Vulkanstudien,"  Leipzig,  1874.  Sartorius  von  Waltershausen  and 
A.  von  Lasaulx,  "Der  Aetna,"  4to,  Leipzig,  1880.  E.  Reyer,  "Beitrag  zur 
Physik  der  Eruptionen,"  Vienna,  1877;  "Die  Euganeen;  Bau  und  Geschichte 
eines  Vulkanes. "  Vienna,  1877.  Fouque,  "Santorin  et  ses  eruptions,"  Paris, 
1879.  Judd,  "Volcanoes,"  1881.  G.  Mercalli,  "Vulcani  e  Fenomeui  vulcanici 
in  Italia,"  Milan,  1883.  Ch.  Velain,  "Les  Volcans,"  Paris,  1884.  J.  D.  Dana, 
"Characteristics  of  Volcanoes,"  1890.  "Volcanoes  Past  and  Present,"  E.  Hull, 
1892.  "The  South  Italian  Volcanoes,"  H.  J.  Johnston -La  vis,  Naples,  1891. 
References  will  be  found  in  succeeding  pages  to  other  and  more  special  memoirs. 


328  TEXT-BOOK   OF   GEOLOGY 

phases  of  volcanism  which  could  not  be  studied  in  any 
modern  volcano.  Hence  an  acquaintance  only  with  active 
volcanoes  will  not  afford  a  complete  knowledge  of  volcanic 
action.  It  must  be  supplemented  and  enlarged  by  an  inves- 
tigation of  the  traces  of  ancient  volcanoes  preserved  in  the 
crust  of  the  earth.  (Book  IV.  Part  VII.) 

The  word  "volcano"  is  applied  to  a  conical  hill  or  moun- 
tain (composed  mainly  or  wholly  of  erupted  materials),  from 
the  summit  and  often  also  from  the  sides  of  which,  hot 
vapors  issue,  and  ashes  and  streams  of  molten  rock  are  in- 
termittently expelled.  The  term  "volcanic"  designates  all 
the  phenomena  essentially  connected  with  one  of  these  chan- 
nels of  communication  between  the  surface  and  the  heated 
interior  of  the  globe.  Yet  there  is  good  reason  to  believe 
that  the  active  volcanoes  of  the  present  day  do  not  afford  by 
any  means  a  complete  type  of  volcanic  action.  The  first 
effort  in  the  formation  of  a  new  volcano  is  to  establish  a 
fissure  in  the  earth's  crust.  A  volcano  is  only  one  vent  or 
group  of  vents  established  along  the  line  of  such  a  fissure. 
But  in  many  parts  of  the  earth,  alike  in  the  Old  World  and 
the  New,  there  have  been  periods  in  the  earth's  history  when 
the  crust  was  rent  into  innumerable  fissures  over  areas  thou- 
sands of  square  miles  in  extent,  and  when  the  molten  rock, 
instead  of  issuing,  as  it  does  at  a  modern  volcano,  in  nar- 
row streams  from  a  central  elevated  cone,  welled  out  from 
numerous  small  vents  along  the  rents,  and  flooded  enormous 
tracts  of  country  without  forming  any  mountain  or  conspicu- 
ous volcanic  cone  in  the  usual  sense  of  these  terms.  Of 
these  "fissure-eruptions,"  apart  from  central  volcanic  cones, 
no  examples  appear  to  have  occurred  within  the  times  of 
human  history,  except  in  Iceland,  where  vast  lava-floods 
issued  from  a  fissure  in  1783  (pp.  378,  434).  They  can  best 


DYNAMICAL    GEOLOGY  329 

be  studied  from  the  remains  of  former  convulsions.  Their 
importance,  however,  has  not  yet  been  generally  recognized 
in  Europe,  though  acknowledged  in  America,  where  they 
have  been  largely  developed.  Much  still  remains  to  be 
done  before  their  mechanism  is  as  well  understood  as  that 
of  the  lesser  type  to  which  all  present  volcanic  action  be- 
longs. In  the  succeeding  narrative  an  account  is  first  pre- 
sented of  the  ordinary  and  familiar  volcano  and  its  products; 
and  in  §  3,  ii.,  some  details  are  given  of  the  general  aspect 
and  character  of  fissure-eruptions. 

The  openings  by  which  heated  materials  from  the  in- 
terior now  reach  the  surface  include  volcanoes  (with  their 
various  associated  orifices)  and  hot-springs. 

The  prevailing  conical  form  of  a  volcano  is  that  which 
the  ejected  materials  naturally  assume  round  the  vent  of 
eruption.  The  summit  of  the  cone  is  truncated  (Figs.  39, 
45),  and  presents  a  cup-shaped  or  caldron-like  cavity,  termed 
the  crater,  at  the  bottom  of  which  is  the  top  of  the  main 
funnel  or  pipe  of  communication  with  the  heated  interior. 
A  volcano,  when  of  small  size,  may  consist  merely  of  one 
cone;  when  of  the  largest  dimensions,  it  forms  a  huge  moun- 
tain, with  many  subsidiary  cones  and  many  lateral  fissures 
or  pipes,  from  which  the  heated  volcanic  products  are  given 
out.  Mount  Etna  (Fig.  39),  rising  from  the  sea  to  a  height 
of  10,840  feet,  and  supporting,  as  it  does,  some  200  minor 
cones,  many  of  which  are  in  themselves  considerable  hills, 
is  a  magnificent  example  of  a  colossal  volcano.4 

2  The  structure  and  history  of  Etna  are  fully  described  in  the  great  work  of 
Sartorius  von  Waltershausen  and  A.  von  Lasaulx  cited  on  p.  327 — a  treasure- 
house  of  facts  in  volcanic  geology.  See  also  G.  F.  Rodwell,  "Etna,  a  history  of 
the  mountain  and  its  eruptions,"  London,  1878;  0.  Silvestri,  "Un  Viaggio  all' 
Etna,"  1879.  Notices  of  recent  eruptions  of  the  mountain  will  be  found  in 
Nature,  vols.  xix.,  xx.,  xxi.,  xxii.,  xxv.  (observatory  on  Etna,  p.  394),  xxvii., 
xlvi. ;  Compt.  rend.  Ixvi.  The  work  of  Mercalli,  cited  on  p.  327,  gives  descrip- 
tions of  this  and  the  other  Italian  volcanic  centres. 


330 


TEXT-BOOK    OF   GEOLOGY 


The  materials  erupted  from  volcanic  vents  may  be  classed 
as  (1)  gases  and  vapors,  (2)  water,  (3)  lava,  (4)  fragmentary 
substances.  A  brief  summary  under  each  of  these  heads 


may  be  given  here;  the  share  taken  by  the  several  products 
in  the  phenomena  of  an  active  volcano  is  described  in  §  2. 
1.  Gases  and  Vapors  exist  dissolved  in  the  molten  magma 


DYNAMICAL    GEOLOGY  331 

within  the  earth's  crust.  They  play  an  important  part  in 
volcanic  activity,  showing  themselves  in  the  earliest  stages 
of  a  volcano's  history,  and  continuing  to  appear  for  centu- 
ries after  all  other  subterranean  action  has  ceased.  By 
much  the  most  abundant  of  them  all  is  water-gas,  which, 
ultimately  escaping  as  steam,  has  been  estimated  to  form 
•Aftnrths  of  the  whole  cloud  that  hangs  over  an  active  volcano 
(Fig.  40).  In  great  eruptions,  steam  rises  in  prodigious 


Fig.  40.— View  of  Vesuvius  as  seen  from  Naples  during  the  eruption  of  1872, 
showing  the  dense  clouds  of  condensed  aqueous  vapor. 

quantities,  and  is  rapidly  condensed  into  a  heavy  rainfall. 
M.  Fouque  calculated  that,  during  100  days,  one  of  th'e 
parasitic  cones  on  Etna  had  ejected  vapor  enough  to  form, 
if  condensed,  2,100,000  cubic  metres  (462,000,000  gallons) 
of  water.  But  even  from  volcanoes  which,  like  the  Sol- 
fatara  of  Naples,  have  been  dormant  for  centuries,  steam 
sometimes  still  rises  without  intermission  and  in  consider- 
able volume.  Jets  of  vapor  rush  out  from  clefts  in  the 


332  TEXT-BOOK    OF   GEOLOGY 

sides  and  bottom  of  a  crater  with  a  noise  like  that  made 
by  the  steam  blown  off  by  a  locomotive.  The  number  of 
these  funnels  or  "fumaroles"  is  often  so  large,  and  the 
a/mount  of  vapor  so  abundant,  that  only  now  and  then, 
when  the  wind  blows  the  dense  cloud  aside,  can  a  momen- 
tary glimpse  be  had  of  a  part  of  the  bottom  of  the  crater; 
while  at  the  same  time  the  rush  and  roar  of  the  escaping 
steam  remind  one  of  the  din  of  some  vast  factory.  Aqueous 
vapor  rises  likewise  from  rents  on  the  outside  of  the  vol- 
canic cone.  It  issues  so  copiously  from  some  flowing  lavas 
that  the  stream  of  rock  may  be  almost  concealed  from  view 
by  the  cloud;  and  it  continues  to  escape  from  fissures  of 
the  lava,  far  below  the  point  of  exit,  for  a  long  time  after 
the  rockrlias  solidified  and  come  to  rest.  So  saturated  are 
many  molten  lavas  with  water-vapor  that  Mr.  Scrope 
thought  that  they  owed  their  mobility  to  this  cause.'  In 
the  deep  volcanic  magma  the  water-substance  must  be  far 
above  its  critical  temperature,  which  is  about  773°  Fahr. 

Probably  in  no  case  is  the  steam  mere  pure  vapor  of 
water,  though  when  it  condenses  into  copious  rain,  it  is 
fresh  and  not  salt  water.  It  is  associated  with  other  vapors 
and  gases  disengaged  from  the  potent  chemical  laboratory 
underneath.  There  seems  to  be  always  a  definite  order  in 
the  appearance  of  these  vapors,  though  it  may  vary  for 
different  volcanoes.  The  hottest  and  most  active  "fuma- 
roles," or  vapor-vents,  may  contain  all  the  gases  and  vapors 
of  a  volcano,  but  as  the  heat  diminishes,  the  series  of  gas- 
eous emanations  is  reduced.  Thus  in  the  Vesuvian  erup- 
tion of  1855-56,  the  lava,  as  it  cooled  and  hardened,  gave 
out  successively  vapors  of  hydrochloric  acid,  chlorides,  and 

8  "Considerations  on  Volcanoes"  (1825),  p.  110. 


DYNAMICAL  GEOLOGY  333 

sulphurous  acid;  then  steam;  and,  finally,  carbon-dioxide 
and  combustible  gases.4  More  recent  observations  tend  to 
corroborate  the  deductions  of  C.  Sainte-Claire  Deville  that 
the  nature  of  the  vapors  evolved  depends  on  the  tempera- 
ture or  degree  of  activity  of  the  volcanic  orifice,  chlorine 
(and  fluorine)  emanations  indicating  the  most  energetic 
phase  of  eruptivity,  sulphurous  gases  a  diminishing  condi- 
tion, and  carbonic  acid  (with  hydrocarbons)  the  dying  out 
of  the  activity.6  A  "solfatara,"  or  vent  emitting  only  gase- 
ous discharges,  is  believed  to  pass  through  these  successive 
stages.  Wolf  observed  that  on  Cotopaxi  while  hydrochloric 
acid  and  even  free  chlorine  escaped  from  the  summit  of  the 
cone,  sulphuretted  hydrogen  and  sulphurous  acid  issued 
from  the  middle  and  lower  slopes.'  Fouque^s  studies  at 
Santorin  have  shown  also  that  from  submarine  vents  a 
similar  order  of  appearance  obtains  among  the  volcanic 
vapors,  hydrochloric  and  sulphurous  acids  being  only  found 
at  points  of  emission  having  a  temperature  above  100°  C., 


4  C.  Sainte-Claire  Deville  and  Leblanc,  Ann.  Chim.  et  Phys.,  1858,  lii.  p.  19 
et  seq.     For  accounts  of  Vesuvius  and  its  eruptions,  besides  the  general  works 
already  cited  on  p.  327,  consult  J.  Phillips'  "Vesuvius,"  1869;  "Mount  Vesu- 
vius," J.  L.  Lobley,  1889;  J.  Schmidt,  "Die  Eruption  des  Vesuv.  1855,"  Vienna, 
1856;  Mercalli's  "Vulcani,  etc.";  H.  J.  Johnston-Lavis,  Q.  J.  Geol.  Soc.  xl.  35; 
Geol.  Mag.  1888,  p.  445.     A  diary  of  the  volcano's  behavior  for  six  months  is 
given  in  Nature,  xxvi. ;  one  for  four  years  (1882-86)  by  Dr.  Johnston-Lavis 
"Spettatore  del  Vesuvio,"  Naples,  1887;  a  valuable  series  of  reports  on  the 
mountain  by  the  same  author  will  be  found  in  recent  volumes  of  the  Reports  of 
the  British  Association  (1885-91)  and  a  large  detailed  map  of  the  volcano,  alpo 
by  him,  published  by  Philip,  London,  1891. 

5  He  distinguished  volcanic  emanations  according  to  their  order  of  appear- 
ance as  regards  time,  nearness  to  the  vent,  and  temperature:  viz.  1.  Dry  fuma- 
roles  (without  steam),  where  anhydrous  chlorides  are  almost  the  only  discharge, 
and  where  the  temperature  is  very  high  (above  that  of  melted  zinc).     2.  Acid 
fumaroles,  with  sulphurous  and  hydrochloric  acids  and  steam.     3.   Alkaline 
(ammoniacal)  fumaroles;  temperature  about  100°  0. ;  abundant  steam  with  chlo- 
ride of  ammouium.     4.  Cold  fumaroles;  temperature  below  100°  C.,  with  nearly 
pure  steam  accompanied  by  a  little  carbon-dioxide,  and  sometimes  sulphuretted 
hydrogen.    5.  Mofettes ;  emanations  of  carbon-dioxide  with  nitrogen  and  oxygen, 
marking  the  last  phase  of  volcanic  activity. 

6  Neues  Jahrb.  1878,  p.  164. 


334  TEXT-BOOK    OF   GEOLOGY 

while  carbon- dioxide,  sulphuretted  hydrogen,  and  nitrogen 
occur  at  all  the  fumaroles,  even  where  the  temperature  is 
not  higher  than  that  of  the  atmosphere.* 

The  following  are  the  chief  gases  and  acids  evolved  at 
volcanic  fumaroles.  Hydrochloric  acid  is  abundant 
at  Vesuvius,  and  probably  at  many  other  vents  whence  it 
has  not  been  recorded.  It  is  recognizable  by  its  pungent, 
suffocating  fumes,  which  make  approach  difficult  to  the 
clefts  from  which  it  issues.  Sulphuretted  hydrogen 
and  sulphurous  acid  are  distinguishable  by  their 
odors.  The  liability  of  the  former  gas  to  decomposition 
leads  to  the  deposition  of  a  yellow  crust  of  sulphur; 
occasionally,  also,  the  production  of  sulphuric  acid  is 
observed  at  active  vents.  From  observations  made  at  Vesu- 
vius in  May,  1878,  Mr.  Siemens  concluded  that  vast  quan- 
tities of  free  hydrogen  or  of  combustible  com- 
pounds of  this  gas  exist  dissolved  in  the  magma  of  the 
earth's  interior,  and  that  these,  rising  and  exploding  in  the 
funnels  of  volcanoes,  give  rise  to  the  detonations  and  clouds 
of  steam.8  At  the  eruption  of  Santorin  in  1866,  the  same 
gases  were  also  distinctly  recognized  by  Fouque,  who  for 
the  first  time  established  the  existence  of  true  volcanic 
flames.  These  were  again  studied  spectroscopically  in  the 
following  year  by  Janssen,  who  found  them  to  arise  essen- 
tially from  the  combustion  of  free  hydrogen,  but  with  traces 
of  chlorine,  soda,  and  copper.  Fouque"  determined  by  analy- 
sis that,  immediately  over  the  focus  of  eruption,  free  hydro- 
gen formed  thirty  per  cent  of  the  gases  emitted,  but  that 
the  proportion  of  this  gas  rapidly  diminished  with  distance 
from  the  active  vents  and  hotter  lavas,  while  at  the  same 
time  the  proportion  of  marsh-gas  and  carbon-dioxide  rapidly 
increased.  The  gaseous  emanations  collected  by  him  were 
found  to  contain  abundant  free  o^  y  g  e  n  as  well  as  hy- 
drogen. One  analysis  gave  the  following  results:  carbon- 
dioxide  0-22,  oxygen  21-11,  nitrogen  21-90,  hydrogen  56-70, 
marsh-gas  0'07=100'00.  This  gaseous  mixture,  on  coming 
in  contact  with  a  burning  body,  at  once  ignites  with  a  sharp 
explosion.  Foucpe'  infers  that  the  water- vapor  of  volcanic 
vents  may  exist  in  a  state  of  dissociation  within  the  mol- 
ten magma  whence  lavas  rise.8  C  a  r  b  o  n-d  i  o  x  i  d  e  rises 

'  "Santorin  et  ses  Eruptions,"  Paris,  1879. 

8  Monatsb.  K.  Preuss.  Akad.  1878,  p.  588. 

9  Fouque,  "Santorin  et  ses  eruptions,"  p.  225. 


DYNAMICAL    GEOLOGY  335 

chiefly  (a)  after  an  eruption  has  ceased  and  the  volcano  re- 
lapses into  quiescence ;  or  (b)  after  volcanic  action  has  other- 
wise become  extinct.  Of  the  former  phase,  instances  are  on 
record  at  Vesuvius  where  an  eruption  has  been  followed 
by  the  emission  of  this  gas  so  copiously  from  the  ground  as 
to  suffocate  hundreds  of  hares,  pheasants,  and  partridges. 
Of  the  second  phase,  good  examples  are  supplied  by  the 
ancient  volcanic  regions  of  the  Eiiel  and  Auvergne,  where 
the  gas  still  rises  in  prodigious  quantities.  Bischof  esti- 
mated that  the  volume  of  carbonic  acid  evolved  in  the 
Brohl  Thai  amounts  to  5,000,000  cubic  feet,  or  300  tons  of 
gas  in  one  day.  Nitrogen,  derived  perhaps  from  the 
decomposition  of  atmospheric  air  dissolved  in  the  water 
which  penetrates  into  the  volcanic  foci,  has  been  frequently 
detected  among  the  gaseous  emanations.  At  Santorin  it 
was  found  to  form  from  4  to  88  per  cent  of  -  the  gas  ob- 
tained from  different  fumaroles.10  Fluorine  and  iodine 
have  likewise  been  noticed. 

With  these  gases  and  vapors  are  associated  many  sub- 
stances which,  sublimed  by  the  volcanic  heat  or  resulting 
from  reactions  among  the  escaping  vapors,  appear  as  Sub- 
limates along  crevices  wherein  they  reach  the  air  and  are 
cooled.  Besides  sulphur,  there  are  several  chlorides 
(particularly  that  of  sodium,  and  less  abundantly  those  of 
potassium,  iron,  copper,  and  lead);  also  free  sulphuric 
acid,  sal-ammoniac,  specular  iron,  cxide  of 
copper,  boracic  acid,  alum,  sulphate  of  lime, 
fe  Is  pars,  pyroxene,  and  other  substances.  Carbonate 
of  soda  occurs  in  large  quantities  among  the  fumaroles  of 
Etna.  Sodium-chloride  sometimes  appears  so  abundantly 
that  wide  spaces  of  a  volcanic  cone,  as  well  as  of  the  newly 
erupted  lava,  are  crusted  with  salt,  which  can  even  be  profit- 
ably removed  by  the  inhabitants  of  the  district.  Consid- 
erable quantities  of  chlorides,  etc.,  may  thus  be  buried 
between  successive  sheets  of  lava,  and  in  long  subsequent 
times  may  give  rise  to  mineral  springs,  as  has  been  sug- 
gested witn  reference  to  the  saline  waters  which  issue  from 
volcanic  rocks  of  Old  Red  Sandstone  and  Carboniferous  age 
in  Scotland."  The  iron-chloride  forms  a  bright  yellow  and 
reddish  crust  on  the  crater  walls,  as  well  as  on  loose  stones 
on  the  slopes  of  the  cone.  Specular  iron,  from  the  decom- 
position of  iron-chloride,  forms  abundantly  as  thin  lamellae 


10  Fouqu6,  loc.  cit  "  Proc.  Ro7-  Soc.  Edin.  ix.  p.  367. 


336  TEXT-BOOK    OF   GEOLOGY 

in  the  fissures  of  Vesuvian  lavas.  In  the  spring  of  1873  the 
author  observed  delicate  brown  filaments  of  tenorite  (copper- 
oxide,  CuO)  forming  in  clefts  of  the  crater  of  Vesuvius. 
They  were  upheld  by  the  upstreaming  current  of  vapor  until 
blown  off  by  the  wind.  Fouque"  has  described  tubular  vents 
in  the  lavas  of  Santorin  with  crystals  of  anorthite,  spheae, 
and  pyroxene,  formed  by  sublimation.  In  the  lava  stalac- 
tites of  Hawaii  needle-like  fibres  of  breislakite  abound. 

2.  Water* — Abundant  discharges  of  water  accompany 
some  volcanic  explosions.  Three  sources  of  this  water  may 
be  assigned :  (1)  from  the  melting  of  snow  by  a  rapid  acces- 
sion of  temperature  previous  to  or  during  an  eruption;  this 
takes  place  from  time  to  time  on  Etna,  in  Iceland,  and 
among  the  snowy  ranges  of  the  Andes,  where  the  cone  of 
Cotopaxi  is  said  to  have  been  entirely  divested  of  its  snow 
in  a  single  night  by  the  heating  of  the  mountain;  (2)  from 
the  condensation  of  the  vast  clouds  of  steam  which  are  dis- 
charged during  an  eruption;  this  undoubtedly  is  the  chief 
source  of  the  destructive  torrents  so  frequently  observed  to 
form  part  of  the  phenomena  of  a  great  volcanic  explosion; 
and  (3)  from  the  disruption  of  reservoirs  of  water  filling  sub- 
terranean cavities,  or  of  lakes  occupying  crater-basins;  this 
has  several  times  been  observed  among  the  South  American 
volcanoes,  where  immense  quantities  of  dead  fish,  which  in- 
habited the  water,  have  been  swept  down  with  the  escaping 
torrents.  The  volcano  of  Agua  in  Guatemala  received  its 
name  from  the  disruption  of  a  crater-lake  at  its  summit  by 
an  earthquake  in  1540,  whereby  a  vast  and  destructive  de- 
bacle of  water  was  discharged  down  the  slopes  of  the  moun- 
tain." In  the  beginning  of  the  year  1817,  an  eruption  took 
place  at  the  large  crater  of  Idjen,  one  of  the  volcanoes  of 


19  For  an  account  of  this  mountain  see  K.  v.  Seebach,  Abh.  Gesell.  Wiai. 
Gottingen,  xxxviii.  (1892)  p.  216. 


DYNAMICAL    GEOLOGY  337 

Java,  whereby  a  steaming  lake  of  hot  acid  water  was  dis- 
charged with  frightful  destruction  down  the  slopes  of  the 
mountain.  After  the  explosion,  the  basin  filled  again  with 
water,  but  its  temperature  was  no  longer  high.18 

In  many  cases,  the  water  rapidly  collects  volcanic  dust 
as  it  rushes  down,  and  soon  becomes  a  pasty  mud;  or  it 
issues  at  first  in  this  condition  from  the  volcanic  reservoirs 
after  violent  detonations.  Hence  arise  what  are  termed 
mud-lavas,  or  aqueous  lavas,  which  in  many  respects 
behave  like  true  lavas.  This  volcanic  mud  eventually  con- 
solidates into  one  of  the  numerous  forms  of  tuff,  a  rock 
which,  as  has  been  already  stated  (p.  238),  varies  greatly  in 
the  amount  of  its  coherence,  in  its  composition,  and  in  its 
internal  arrangement.  Obviously,  unless  where  subsequently 
altered,  it  cannot  possess  a  crystalline  structure  like  that  of 
true  lava.  As  a  rule,  it  betrays  its  aqueous  origin  by  more 
or  less  distinct  evidence  of  stratification,  by  the  multifarious 
pebbles,  stones,  blocks  of  rock,  tree-trunks,  branches,  shells, 
bones,  skeletons,  etc.,  which  it  has  swept  along  in  its  Bourse 
and  preserved  within  its  mass.  Sections  of  this  compacted 
tuff  may  be  seen  at  Herculaneum.14  The  trass  of  the  Brohl 
Thai  and  other  valleys  in  the  Eifel  district,  referred  to  on 
p.  242,  is  another  example  of  an  ancient  volcanic  mud. 

13  See  Junghuhn's  "Java."     For  an  account  of  the  volcanoes  of  the  Sunda 
Island  and  Moluccas,  see  F.  Scheider,  Jahrb.  Geol.  Reichsanst.  Vienna,  xxxv. 
(1885),  p.  1.     Consult  also  for  the  Javanese  volcanoes  the  works  on  Krakfctoa 
quoted  on  p.  362. 

14  Mallet  thought  that  the  so-called  "mud-lavas"  of  Herculaneum  and  Pom- 
peii were  not  aqueous  deposits  (Journ.  Roy.  Geol.  Soc.  Ireland,  IV.  (1876),  p. 
144).     But  theie  seems  no  reason  to  doubt  that  while  an  enormous  amount  of 
ashes  fell  during  the  eruption  of  A.D.  79,  there  were  likewise,  especially  in  the 
later  phases  of  eruption,  copious  tori  cuts  of  water  that  mingled  with  the  line  ash 
and  became  "mud-lavas."     The  sharpness  of  outline  and  the  absence  of  any 
trace  of  abdominal  distension  in  the  molds  of  the  human  bodies  found  at  Pom- 
peii, probably  show  that,  these  victims  of  the  catastrophe  were  rapidly  enveloped 
in  a  firm  coherent  matrix  which  could  hardly  have  been  mere  loose  dual.     See 
H.  J.  Johnstou-Lavis,  Q.  J.  Geol.  Soc.  xl.  p.  89. 

GEOLOGY— Vol.  XXIX— 15 


338  TEXT-BOOK    OF   GEOLOGY 

3.  Lava.  —The  term  lava  is  applied  generally  to  all  the 
molten  rocks  of  volcanoes.1*  The  use  of  the  word  in  this 
broad  sense  is  of  great  convenience  in  geological  descrip- 
tions, by  directing  attention  to  the  leading  character  of  the 
rocks  as  molten  products  of  volcanic  action,  and  obviating 
the  confusion  and  errors  which  are  apt  to  arise  from  an  ill- 
defined  or  incorrect  lithological  terminology.  Precise  defi- 
nitions of  the  rocks,  such  as  those  given  above  in  Book  II., 
can  be  added  when  required.  A  few  remarks  regarding 
some  of  the  general  lithological  characters  of  lavas  may  be 
of  service  here;  the  behavior  of  the  rocks  in  their  emission 
from  volcanic  orifices  will  be  described  in  §  2. 

While  still  flowing  or  not  yet  cooled,  lavas  differ  from 
each  other  in  the  extent  to  which  they  are  impregnated  with 
gases  and  vapors.  Some  appear  to  be  saturated,  others  con- 
tain a  much  smaller  gaseous  impregnation;  and  hence  arise 
important  distinctions  in  their  behavior  (pp.  370-395).  After 
solidification,  lavas  present  some  noticeable  characters,  then 
easily  ascertainable.  (1)  Their  average  specific  gravity  may 
be  tak«n  as  ranging  between  2-37  and  3-22.  (2)  The  heavier 
varieties  contain  much  magnetic  or  titaniferous  iron,  with 
augite  and  olivine,  their  composition  being  basic,  and  their 
proportion  of  silica  averaging  about  45  to  55  per  cent.  In 
this  group  come  the  basalts,  nepheline-lavas,  and  leucite- 
lavas.  The  lighter  varieties  contain  commonly  a  minor  pro- 
portion of  metallic  bases,  but  are  rich  in  silica,  their  per- 
centage of  that  acid  ranging  between  70  and  75.  They  are 
thus  not  basic  but  acid  rocks.  Among  their  more  important 
varieties  are  the  rhyolites  and  obsidians.  Some  intermedi- 
ate varieties  (trachytes,  phonolites,  and  andesites)  connect 
the  acid  and  basic  series.  (3)  Lavas  differ  much  in  structure 
and  texture,  (a)  Some  are  entirely  crystalline,  consisting 
of  an  interlaced  mass  of  crystals  and  crystalline  particles,  as 
in  some  dolerites,  and  granitoid  rhyolites.  Even  quartz, 
which  used  to  be  considered  a  non-volcanic  mineral,  charac- 


15  "All  es  ist  Lava  was  im  Vulkane  fliesst  und  durch  seine  Flussigkeit  neue 
Lagerstatter  einnimmt"  is  Leopold  von  Buch's  comprehensive  definition. 


DYNAMICAL    GEOLOGY  339 

teristic  of  the  older  and  chiefly  of  the  plutonic  eruptive 
rocks,  has  been  observed  in  large  crystals  in  modern  lava 
(liparite  and  quartz-andesite").  (6)  Some  show  more  or  less 
of  a  half-glassy  or  stony  (devitrified)  matrix,  in  which  the 
constituent  crystals  are  imbedded;  this  is  the  most  common 
arrangement,  (c)  Others  are  entirely  vitreous,  such  crystals 
or  crystalline  particles  as  occur  in  them  being  c[uite  subor- 
dinate, and,  so  to  speak,  accidental  inclosures  in  the  main 
glassy  mass.  Obsidian  or  volcanic  glass  is  the  type  of  this 
group,  (d)  They  further  differ  in  the  extent  to  which  mi- 
nute pores  or  larger  cellular  spaces  have  been  developed  in 
them.  According  to  Biscbof,  the  porosity  of  lavas  depends 
on  their  degree  of  liquidity,  a  porous  lava  or  slag,  when  re- 
duced in  his  fusion-experiments  to  a  thin-flowing  consist- 
ency, hardening  into  a  mass  as  compact  as  the  densest  lava 
or  basalt.17  The  presence  of  interstitial  steam  in  lavas,  by 
expanding  the  still  molten  stone,  produces  an  open  cellular 
texture,  somewhat  like  that  of  sponge  or  of  bread.  Such  a 
vesicular  arrangement  very  commonly  appears  on  the  upper 
surface  of  a  lava  current,  which  assumes  a  slaggy  or  cindery 
aspect.  In  some  forms  of  purnice  the  proportion  of  air 
cavities  is  8  or  9  times  that  of  the  inclosing  glass.  (4)  Lavas 
vary  greatly  in  color  and  general  external  aspect.  The 
heavy  basic  kinds  are  usually  dark  gray,  or  almost  black, 
though,  on  exposure  to  the  weather,  they  acquire  a  brown 
tint  from  the  oxidation  and  hydration  of  their  iron.  Their 
surface  is  commonly  rough  and  ragged,  until  it  has  been 
sufficiently  decomposed  by  the  atmosphere  to  crumble  into 
soil  which,  under  favorable  circumstances,  supports  a  luxu- 
riant vegetation.  The  less  dense  lavas,  such  as  phonolites 
and  trachytes,  are  frequently  paler  in  color,  sometimes  yel- 
low or  buff,  and  decompose  into  light  soils;  but  the  obsid- 
ians present  rugged  black  sheets  of  rock,  roughened  with 
ridges  and  heaps  of  gray  froth-like  pumice.  Some  of  the 
most  brilliant  surfaces  of  color  in  any  rock-scenery  on  the 
globe  are  to  be  found  among  volcanic  rocks.  The  walls  of 
active  craters  glow  with  endless  hues  of  red  and  yellow. 
The  Grand  Oaflon  of  the  Yellowstone  Eiver  has  been  dug 
out  of  the  most  marvellously  tinted  lavas  and  tuffs. 


16  Wolf,  Neues  Jahrb.  1874,  p.  377. 

11  "Chem.  und  Phys.  Geol."  supp.  (1871),  p.  144.  On  the  production  of  the 
vesicular  structure  consult  Dana,  "Characteristics  of  Volcanoes,"  p.  161.  Com- 
pare also  Judd,  Geol.  Mag.  1888,  p.  7. 


S40  TEXT-BOOK   OF   GEOLOGY 

4.  Fragmentary  Materials.— Under  this  title  may  be  in- 
cluded all  the  substances  which,  driven  up  into  the  air  by 
volcanic  explosions,  fall  in  solid  form  to  the  ground — the 
dust,  ashes,  sand,  cinders,  and  blocks  of  every  kind  which 
are  projected  from  a  volcanic  orifice.  These  materials  differ 
in  composition,  texture,  and  appearance,  even  during  a  sin- 
gle eruption,  and  still  more  in  successive  explosions  of  the 
same  volcano.  For  the  sake  of  convenience,  separate  names 
are  applied  to  some  of  the  more  distinct  varieties,  of  which 
the  following  may  be  enumerated. 

(1)  Ashes  and  san  d. — In  many  eruptions,  vast  quan- 
tities of  an  exceedingly  fine  light  gray  powder  are  ejected. 
As  this  substance  greatly  resembles  what  is  left  after  a  piece 
of  wood  or  coal  is  burned  in  an  open  fire,  it  has  been  popu- 
larly termed  ash,  and  this  name  has  been  adopted  by  geolo- 
gists.    If,  however,  by  the  word  ash,  the  result  of  combus- 
tion is  implied,  its   employment   to  denote  any  product  of 
volcanic  action  must  be  regretted,  as  apt  to  convey  ;i  wrong 
impression.     The  fine  ash-like  dust  ejected  by  a  volcano  is 
merely  lava  in  an  extremely  fine  state  of  comminution.     So 
minute  are  the   particles   that  they  find   their  way  readily 
through  the  finest  chinks  of  a  closed  room,  and  settle  down 
upon  floor  and  furniture,  as  ordinary  dust  does  when  a  house 
is  shut  up.     From   this  finest  form  of  material,  gradations 
may  be  traced,  through  what  is  termed  volcanic  sand,  into 
the  coarser  varieties  of  ejected  matter.     In  composition,  the 
ash  and  sand  vary  necessarily  with  the  nature  of  the  lava 
from  which  they  are  derived.     Their  microscopic  structure, 
and  especially  their  abundant  microlites,  crystals,  and  vol- 
canic glass,  have  been  already  referred  to  (pp.  239-241). 

(2)  L  a  p  i  1 1  i  or  r  a  p  i  1 1 1  (p.  239)  are  Dejected  fragments 
ranging  from   the  size  of  a  pea  to  that  of*  a  walnut;  round, 
subangular,  or  angular  in  shape,  and  having  the  same  indefi- 
nite range  of  composition  as  the  finer  dust.     As  a  rule,  the 
larger  pieces  fall  nearest  the  focus  of  eruption.     Sometimes 
they  are  solid  fragments  of  lava,  but  more  usually  they  have 
a  cellular  texture,   while  sometimes  they  are  so  light  and 
porous  as  to  float  readily  on  water,  and,  when  ejected  near 
the  sea,  to  cover  its  surface.      Well  formed  crystals  occur  in 
the  lapilli  of  many  volcanoes,  and  are  also  ejected  separately. 


DYNAMICAL    GEOLOGY  341 

It  has  been  observed  indeed  that  the  fragmentary  materials 
not  infrequently  contain  finer  crystals  than  the  accompany- 
ing lava.18 

(3)  Volcanic  Blocks  (p.  239)  are  larger  pieces  of 
stone,  often  angular  in  shape.  In  some  cases  they  appear 
to  be  fragments  loosened  from  already  solidified  rocks  in  the 
chimney  of  the  volcano.  Hence  we  find  among  them  pieces 
of  non-volcanic  rocks,  as  well  as  of  older  tuffs  and  lavas  rec- 
ognizably belonging  to  early  eruptions.  In  many  cases, 
they  are  ejected  in  enormous  quantities  during  the  earlier 
phases  of  violent  eruption.  The  great  explosion  from  the 
side  of  Ararat  in  1840  was  accompanied  by  the  discharge  of 
a  vast  quantity  of  fragments  over  a  space  of  many  square 
miles  around  tne  mountain.  Whitney  has  described  the  oc- 
currence in  California  of  beds  of  such  fragmentary  volcanic 
breccia,  hundreds  of  feet  thick  and  covering  many  square 
miles  of  surface.  Junghuhn,  in  his  account  of  the  eruption 
in  Java  in  1772,  mentions  that  a  valley  ten  miles  long  was 
filled  to  an  average  depth  of  fifty  feet  with  angular  volcanic 
de'bris." 

Among  the  earlier  eruptions  of  a  volcano,  fragments  of 
the  rocks  through  which  the  vent  has  been  drilled  may  fre- 
quently be  observed.  These  are  in  many  cases  not  volcanic. 
Blocks  of  schist  and  granitoid  rocks  occur  in  the  cinder-beds 
at  the  base  of  the  volcanic  series  of  Santorin.  In  the  older 
tuffs  of  Somma,  pieces  of  altered  limestone  (sometimes 
measuring  200  cubic  feet  or  more  and  weighing  upward 
of  15  tons)  are  abundant  and  often  contain  cavities  lined 
with  the  characteristic  "Yesuvian  minerals."80  Blocks  of 
a  coarsely  crystalline  granitoid  (but  really  trachytic)  lava 
have  been  particularly  observed  both  on  Etna"  and  Vesu- 
vius. In  the  year  1870  a  mass  of  that  kind,  weighing  several 
tons,  was  to  oe  seen  lying  at  the  foot  of  the  upper  cone  of 
Vesuvius,  within  the  entrance  to  the  Atrio  del  Cavallo. 
Similar  blocks  occur  among  the  Carboniferous  volcanic 
pipes  of  central  Scotland,  together  sometimes  with  frag- 
ments of  sandstone,  shale,  or  limestone,  not  infrequently 
full  of  Carboniferous  fossils."  Enormous  masses  of  various 


18  Sartorius  von  Waltershausen,  "Sicilien  und  Island,"  1853,  p.  328. 

19  But  see  the  remarks  already  made  on  volcanic  conglomerate,  ante.  p.  283. 

20  See  H.  J.  Johnston -Lavis,  Q.  J.  Geol.  Soc.  xl.  p.  76. 

21  For  the  erupted  blocks  (Answurflinge)  of  Etna  see  "Der  Aetna,"  ii.  pp. 
216,  330,  461. 

82  Trans.  Roy.  Soc.  Edin.  xxix.  p.  469.    See  postea,  Book  IT.  Sect.  vii.  §  1,  4. 


342  TEXT-BOOK    OF   GEOLOGY 

schists  have  been  carried  up  by  the  lavas  of  the  Tertiary 
volcanic  plateau  of  the  Inner  Hebrides.28 

(4)  Volcanic  Bombs  and  slags. — These  have  orig- 
inally formed  portions  of  the  column  of  lava  ascending  the 
pipe  of  a  volcano,  and  have  been  detached  and  hurled  into 
the  air  by  successive  explosions  of  steam.  A  bomb  (Fig. 
41)  is  a  round,  elliptical,  or  pear-shaped,  often  discoidal 
mass  of  lava,  from  a  few  inches  to  several  feet  in  diameter; 
sometimes  tolerably  solid  throughout,  more  usually  coarsely 
cellular  inside.  Not  infrequently  its  interior  is  hollow,  and 
the  bomb  then  consists  of  a  shell  which  is  most  close-grained 


Fig.  41.— Section  of  Volcanic  Bomb,  one-third  natural  size. 

toward  the  outside,  or  the  centre  is  a  block  of  stone  with  an 
external  coating  of  lava.  There  can  be  no  doubt  that,  when 
torn  by  eructations  of  steam  from  the  surface  of  the  boiling 
lava,  the  material  of  these  bombs  is  in  as  thoroughly  molten 
a  condition  as  the  rest  of  the  mass.  From  the  rotatory  mo- 
tion imparted  by  its  ejection,  it  takes  a  circular  form,  and 
in  proportion  to  its  rapidity  of  rotation  and  fluidity  is  the 
amount  of  its  "flattening  at  the  poles."  The  centrifugal 
force  within  allows  the  expansion  of  the  interstitial  vapor, 
while  the  outer  surface  rapidly  cools  and  solidifies;  hence 
the  solid  crust,  and  the  porous  or  cavernous  interior.  Such 
bombs,  varying  from  the  size  of  an  apple  to  that  of  a  man's 
body,  were  found  by  Darwin  abundantly  strewn  over  the 
ground  in  the  Island  of  Ascension;  they  were  also  ejected 

23  Trans.  Roy.  Soc.  Ediu.  xxxv.  (1888),  p.  82. 


DYNAMICAL    GEOLOGY  343 

in  vast  quantities  during  the  eruption  of  Santorin  in  1866." 
Among  the  tuffs  of  the  Eifel  region,  small  bombs,  consist- 
ing mostly  of  granular  olivine,  are  of  common  occurrence, 
as  also  pieces  of  sanidine  or  other  less  fusible  minerals 
which  have  segregated  out  of  the  magma  before  ejection. 
In  like  manner,  among  the  tuffs  filling  volcanic  necks, 
probably  of  Permian  age,  which  pierce  the  Carboniferous 
rocks  of  Fife,  large  worn  crystals  of  orthoclase,  biotite,  etc., 
are  found.  When  the  ejected  fragment  of  lava  has  a  rough 
irregular  form  and  a  porous  structure,  like  the  clinker  of 
an  iron  furnace,  it  is  known  as  a  slag." 

The  fragmentary  materials  erupted  by  a  volcano  and  de- 
posited around  it  acquire  by  degrees  more  or  less  consolida- 
tion, partly  from  the  mere  pressure  of  the  higher  upon  the 
lower  strata,  partly  from  the  influence  of  infiltrating  water. 
It  has  been  already  stated  (p.  240)  that  different  names  are 
applied  to  the  rocks  thus  formed.  The  coarse,  tumultuous, 
unstratified  accumulation  of  volcanic  de'bris  within  a  crater 
or  funnel  is  called  Agglomerate.  When  the  debris, 
though  still  coarse,  is  more  rounded,  and  is  arranged  in  a 
stratified  form  on  the  slopes  of  the  cone  or  on  the  country 
beyond,  it  becomes  aYolcanic  Conglomerate.  The 
finer-grained  varieties,  formed  of  dust  and  lapilli,  are  in- 
cluded in  the  general  designation  of  Tuffs.  These  are 
usually  pale  yellowish,  grayish,  or  brownish,  sometimes 
black  rocks,  granular,  porous,  and  often  incoherent  in  tex- 
ture. They  occur  interstratified  with  and  pass  into  ordinary 
non-volcanic  sediment. 

Organic  remains  sometimes  occur  in  tuff.  Where  vol- 
canic debris  has  accumulated  over  the  floor  of  a  lake,  or  of 
the  sea,  the  entombing  and  preserving  of  shells  and  other 
organic  objects  must  continually  take  place.  Examples  of 
this  kind  are  cited  in  later  pages  of  this  work  from  older 
geological  formations.  Professor  Guiscardi  of  Naples  found 
about  100  species  of  marine  shells  of  living  species  in  the 
old  tuffs  of  Vesuvius.  Marine  shells  have  been  picked  up 
within  the  crater  of  Monte  Niiovo,  and  have  been  frequently 
observed  in  the  old  or  marine  tuff  of  that  district.  Showers 
of  ash,  or  sheets  of  volcanic  mud,  often  preserve  land-shells, 
insects,  and  vegetation  living  on  the  area  at  the  time.  The 


24  Darwin,  "Geological  Observations  on  Volcanic  Islands,"  2d  edit.  p.  42. 
Fouque,  "Santoriu,"  p.  79. 

25  On  the  ratio  between  the  pores  and  volume  of  the  rock  in  slags  and  lavas, 
see  determinations  by  Biscliof,  "Chem.  imd  Phys.  Geol."  supp.  (1871),  p.  158. 


344  TEXT-BOOK   OF   GEOLOGY 

older  tuffs  of  Vesuvius  have  yielded  many  remains  of  the 
shrubs  and  trees  which  at  successive  periods  have  clothed 
the  flanks  of  the  mountain.  Fragments  of  coniferous  wood, 
which  once  grew  on  the  tuff-cones  of  Carboniferous  age  in 
central  Scotland,  are  abundant  in  the  "necks"  of  that  re- 
gion, while  the  minute  structure  of  some  of  the  lepidoden- 
droid  plants  has  also  been  admirably  preserved  there  in 
tuff." 

§2.  Volcanic   Action 

Volcanic  action  many  be  either  constant  or  periodic. 
Stromboli,  in  the  Mediterranean,  so  far  as  we  know,  has 
been  uninterruptedly  emitting  hot  stones  and  steam,  from 
a  basin  of  molten  lava,  since  the  earliest  period  of  history." 
Among  the  Moluccas,  the  volcano  Sioa,  and  in  the  Friendly 
Islands,  that  of  Tofua,  have  never  ceased  to  be  in  eruption 
since  their  first  discovery.  The  lofty  cone  of  Sangay,  among 
the  Andes  of  Quito,  is  always  giving  off  hot  vapors;  Goto- 
paxi,  too,  is  ever  constantly  active.88  But,  though  exam- 
ples of  unceasing  action  may  thus  be  cited  from  widely 
different  quarters  of  the  globe,  they  are  nevertheless  excep- 
tional. The  general  rule  is  that  a' volcano  breaks  out  from 
time  to  time  with  varying  vigor,  and  after  longer  or  shorter 
intervals  of  quiescence. 

Active,  Dormant,  and  Extinct  Phases,— It  is  usual  to  class 
volcanoes  as  active,  dormant,  and  extinct.  This  arrangement, 
however,  often  presents  considerable  difficulty  in  its  appli- 
cation. An  active  volcano  cannot  of  course  be  mistaken, 
for  even  when  not  in  eruption,  it  shows  by  its  discharge  of 

86  Trans.  Roy.  Soc.  Edin.  xxix.  p.  470;  postea,  Book  IV.  Part  VII.  Sect, 
ii.  §  2. 

91  For  accounts  of  Stromboli  see  Spallanzani's  "Voyages  dans  les  deux 
Siciles."  Scrope's  "Volcanoes."  Judd,  Geol.  Mag.  1875.  Mercalli's  "Vul- 
cani,"  etc.  p.  135;  and  his  papers  in  Atti  Soc.  Ital.  Sci.  Nat.  xxii.,  xxiv.,  xxvii., 
xxix.,  xxxi.  L.  "W.  Pulcher,  Geol.  Mag.  1890,  p.  347. 

28  For  descriptions  of  Cotopaxi,  see  Wolf,  Neues  Jahrb.  1878;  Whymper, 
Nature,  xxiii.  p.  323;  "Travels  amongst  the  Great  Andes,"  chap.  vi. 


DYNAMICAL    GEOLOGY  345 

steam  and  hot  vapors  that  it  might  break  out  into  activity 
at  any  moment.  But  in  many  cases,  it  is  impossible  to  de- 
cide whether  a  volcano  should  be  called  extinct  or  only 
dormant.  The  volcanoes  of  Silurian  age  in  Wales,  of  Car- 
boniferous age  in  Ireland,  of  Permian  age  in  the  Harz,  of 
Miocene  age  in  the  Hebrides,  of  younger  Tertiary  age  in 
the  Western  States  and  Territories  of  North  America,  are 
certainly  all  extinct.  Bat  the  older  Tertiary  volcanoes 
of  Iceland  are  still  represented  there  by  Skaptar-Jokull, 
Hecla,  and  their  neighbors."  Somma,  in  the  first  century 
of  the  Christian  era,  would  have  been  naturally  regarded 
as  an  extinct  volcano.  Its  fires  had  never  been  known  to 
have  been  kindled;  its  vast  crater  was  a  wilderness  of  wild 
vines  and  brushwood,  haunted,  no  doubt,  by  wolf  and 
wild  boar.  Yet  in  a  few  days,  during  the  autumn  of  the 
year  79,  the  half  of  the  crater  walls  was  blown  out  by  a 
terrific  series  of  explosions,  the  present  Vesuvius  was  then 
formed  within  the  limits  of  the  earlier  crater,  and  since  that 
time  volcanic  action  has  been  intermittently  exhibited  up 
to  the  present  day.  Some  of  the  intervals  of  quietude, 
however,  have  been  so  considerable  that  the  mountain 
might  then  again  have  been  claimed  as  an  extinct  vol- 
cano. Thus,  in  the  131  years  between  1500  and  1631,  so 
completely  had  eruptions  ceased  that  the  crater  had  once 


89  On  the  volcanic  phenomena  of  Iceland  consult  G.  Mackenzie's  "Travels  in 
the  Island  of  Iceland  during  the  Summer  of  1810."  E.  Henderson's  "Iceland." 
Zirkel,  "De  geognostica  Islandse  constitutione  observationes, "  Bonn,  1861.  Tho- 
roddsen,  "Oversigt  over  de  islandske  Vulkaners  Historie,"  translated  in  resume 
by  G.  H.  Boehmer,  Smithsonian  Inst.  Rep.  1885,  part  i.  p.  495;  also  Bihang  t. 
Svensk.  Yet.  Akad.  Handl.  14,  ii.  (1888),  17,  ii.  (1891);  Geol.  Mag.  1880,  p. 
458;  Nature,  .Oct.  1884.  Mitth.  K.  K.  Geogr.  Ges.  Vienna,  xxiv.  (1891),  p. 
117.  Keilhack,  Zeitsch.  Deutsch.  Geol.  Gesel.  xzxviii.  (1886),  p.  376;  Schmidt, 
op.  cit.  xxxvii.  (1885),  p.  737;  A.  Holland,  "Lakis  Kratere  og  Lava-strome, " 
Universitets  Programme,  Christiania,  1885 ;  Breon,  "Geologic  de  1'Islande,  et  des 
UesFoeroe,"  Paris,  1884;  T.  Anderson,  Journ.  Soc.  Arts,  vol.  xl.  (1892),  p.  397. 


346  TEXT-BOOK   OF   GEOLOGY 

more  become  choked  with  copsewood.  A  few  pools  and 
springs  of  very  salt  and  hot  water  remained  as  memorials 
of  the  former  condition  of  the  mountain.  But  this  period 
of  quiescence  closed  with  the  eruption  of  1631 — the  most 
powerful  of  all  the  known  explosions  of  Vesuvius,  except 
the  great  one  of  79.  In  the  island  of  Ischia,  Mont'  Epomeo 
was  last  in  eruption  in  the  year  1302,  its  previous  outburst 
having  taken  place,  it  is  believed,  about  seventeen  centuries 
before  that  date.  From  the  craters  of  the  Eifel,  Auvergne, 
the  Yivarais,  and  central  Italy,  though  many  of  them  look 
as  if  they  had  only  recently  been  formed,  no  eruption  has 
been  known  to  come  during  the  times  of  human  history  or 
tradition.  In  the  west  of  North  America,  from  Arizona  to 
Oregon,  numerous  stupendous  volcanic  cones  occur,  but 
even  from  the  most  perfect  and  fresh  of  them  nothing  but 
steam  and  hot  vapors  has  yet  been  known  to  proceed.30 
But  the  presence  there  of  hot  springs  and  geysers  proves 
the  continued  existence  of  one  phase  of  volcanic  action. 

In  short,  no  essential  distinction  can  be  drawn  between 
dormant  and  extinct  volcanoes.  Volcanic  action,  as  will  be 
afterward  pointed  out,  is  apt  to  show  itself  again  and  again, 
even  at  vast  intervals,  within  the  same  regions  and  over  the 
same  sites.  The  dormant  or  waning  condition  of  a  volcano, 
when  only  steam  and  various  gases  and  sublimates  are  given 
off,  is  sometimes  called  the  Solfatara  phase,  from  the  well- 
known  dormant  crater  of  that  name  near  Naples. 

Sites  of  Volcanic  Action,— Volcanoes  may  break  through 
any  geological  formation.  In  Auvergne,  in  the  Miocene 
period,  they  burst  through  the  granitic  and  gneissose 
plateau  of  central  France.  In  Lower  Old  Bed*  Sandstone 

80  Eruptions  occurred  perhaps  less  than  100  years  ago.  Diller,  Bull.  U.  S. 
Geol.  Surv.,  No.  79. 


DYNAMICAL    GEOLOGY  347 

times,  they  pierced  contorted  Silurian  rocks  in  central 
Scotland.  In  late  Tertiary  and  post-Tertiary  ages,  they 
found  their  way  through  recent  soft  marine  strata,  and 
formed  the  huge  piles  of  Etna,  Somma,  and  Vesuvius; 
while  in  North  America,  during  the  same  cycle  of  geolog- 
ical time,  they  flooded  with  lava  and  tuff  many  of  the  river- 
courses,  valleys,  and  lakes  of  Nevada,  Utah,  Wyoming, 
Idaho,  and  adjacent  territories.  On  the  banks  of  the  Rhine, 
at  Bonn  and  elsewhere,  they  have  penetrated  some  of  the 
older  alluvia  of  that  river.  In  many  instances,  also,  newer 
volcanoes  have  appeared  on  the  sites  of  older  ones.  In 
Scotland,  the  Carboniferous  volcanoes  have  risen  on  the 
ruins  of  those  of  the  Old  Red  Sandstone,  those  of  the 
Permian  period  have  broken  out  among  the  earlier  Car- 
boniferous eruptions,  while  the  older  Tertiary  dikes  have 
been  injected  into  all  these  older  volcanic  masses.  The 
newer  puys  of  Auvergne  were  sometimes  erupted  through 
much  older  and  already  greatly  denuded  basalt-streams. 
Somma  and  Vesuvius  have  risen  out  of  the  great  Neapoli- 
tan plain  of  older  marine  tuff,  while  in  central  Italy  newer 
cones  have  been  thrown  up  upon  the  wide  Roman  plain  of 
more  ancient  volcanic  de'bris.31  The  vast  Snake  River  lava- 
fields  of  Idaho  overlie  denuded  masses  of  earlier  trachytic 
lavas,  and  similar  proofs  of  a  long  succession  of  intermit- 
tent and  widely-separated  volcanic  outbursts  can  be  traced 
northward  into  the  Yellowstone  Valley. 

When  a  volcanic  vent  is  opened,  it  might  be  supposed 
always  to  find  its  way  to  the  surface  along  some  line  of  fis- 


31  According  to  Prof.  Gr.  Pozzi,  the  principal  volcanic  outbursts  of  Italy  are 
of  the  Glacial  Period.  Atti  Lincei,  3d  ser.  vol.  ii.  (1878),  p.  35.  Stefani  re- 
gards those  of  Tuscany  as  partly  Miocene,  partly  Pliocene  and  post- Pliocene. 
(Proc.  Tosc.  Soc.  Nat.  Pisa,  1.  p.  xxi.) 


348  TEXT-BOOK    OF    GEOLOGY 

sure,  valley,  or  deep  depression.  No  doubt  many,  if  not 
most,  modern  as  well  as  ancient  vents,  especially  those  of 
large  size,  have  done  so.  It  is  a  curious  fact,  however,  that 
in  innumerable  instances  minor  vents  have  appeared  where 
there  was  no  visible  line  of  dislocation  in  the  rocks  at  the 
surface  to  aid  them.  This  has  been  well  shown  by  a  study 
of  the  ancient  volcanic  rocks  of  the  Old  Red  Sandstone,  Car- 
boniferous, and  Permian  formations  of  Scotland.88  It  has 
likewise  been  most  impressively  demonstrated  by  the  way  in 
which  the  minor  basalt  cones  and  craters  of  Utah  have 
broken  out  near  the  edges  or  even  from  the  face  of  cliffs, 
rather  than  at  the  bottom.  Captain  Button  remarks  that 
among  the  high  plateaus  of  Utah,  where  there  are  hundreds 
of  basaltic  craters,  the  least  common  place  for  them  is  at  the 
base  of  a  cliff,  and  that,  though  they  occur  near  faults,  it  is 
almost  always  on  the  lifted,  rarely  upon  the  depressed  side.'8 
On  a  small  scale,  a  similar  avoidance  of  the  valley  bottom  is 
shown  on  the  Rhine  and  Moselle,  where  eruptions  have 
taken  place  close  to  the  edge  of  the  plateau  through  which 
these  rivers  wind.  Why  outbreaks  should  have  occurred  in 
this  way  is  a  question  not  easily  answered.  It  suggests  that 
the  existing  depressions  and  heights  of  the  earth's  surface 
may  sometimes  be  insignificant  features,  compared  with  the 
depth  of  the  sources  of  volcanoes  and  the  force  employed  in 
volcanic  eruption.  On  the  other  hand,  it  is  remarkable  that 
in  Scotland  the  Palaeozoic  eruptions  took  place  on  the  low 
ground  and  valleys,  and  continued  to  show  themselves  there 
during  a  long  succession  of  volcanic  periods.  Especially 
noteworthy  is  the  way  in  which  the  Permian  vents  were 


88  Trans.  Roy.  Soc.  Edin.  xxix.  p.  437. 

33  "High  Plateaus  of  Utah,"  Geol.  and  Geog.  Survey  of  Territories,  1880, 
p.  62. 


DYNAMICAL    GEOLOGY  349 

opened  in  lines  and  groups  along  the  bottom  of  long  narrow 
valleys  in  the  Silurian  uplands.34 

Ordinary  phase  of  an  active  Volcano.— The  interval  be- 
tween two  eruptions  of  an  active  volcano  shows  a  gradual 
augmentation  of  energy.  The  crater,  emptied  by  the  last 
discharge,  has  its  floor  slowly  upraised  by  the  expansive 
force  of  the  lava-column  underneath.  Yapors  rise  in  con- 
stant outflow,  accompanied  sometimes  by  discharges  of  ^dust 
or  stones.  Through  rents  in  the  crater-floor  red-hot  lava 
may  be  seen  only  a  few  feet  down.  Where  the  lava  is  main- 
tained at  or  above  its  fusion-point  and  possesses  great  liquid- 
ity, it  may  form  boiling  lakes,  as  in  the  great  crater  of 
Kilauea,  where  acres  of  seething  lava  may  be  watched  throw- 
ing up  fountains  of  molten  rock,  surging  against  the  walls 
and  re-fusing  large  masses  that  fall  into  the  burning  flood. 
The  lava-column  inside  the  pipe  of  a  volcano  is  all  this  time 
gradually  rising,  until  some  weak  part  of  the  wall  allows  it 
to  escape,  or  until  the  pressure  of  the  accumulated  vapors 
becomes  great  enough  to  burst  through  the  hardened  crust 
of  the  crater-floor  and  give  rise  to  the  phenomena  of  an 
eruption. 

Conditions  of  Eruption* — Leaving  for  the  present  the  gen- 
eral question  of  the  cause  of  volcanic  action,  it  may  be  here 
remarked  that  the  conditions  determining  any  particular 
eruption  are  still  unknown.  The  explosions  of  a  volcano 
may  be  to  some  extent  regulated  by  the  conditions  of  atmos- 
pheric pressure  over  the  area  at  the  time.  In  the  case  of  a 
volcanic  funnel  like  Stromboli,  where,  as  Scrope  pointed 
out,  the  expansive  subterranean  force  within,  and  the  re- 
pressive effect  of  atmospheric  pressure  without,  just  balance 

34  Quart.  Journ.  Geol.  Soc.  vol.  xlviii.  (1892).     Presidential  Address,  p.  156. 


350  TEXT-BOOK   OF   GEOLOGY 

each  other,  any  serious  disturbance  of  that  pressure  might 
be  expected  to  make  itself  evident  by  a  change  in  the  con- 
dition of  the  volcano.  Accordingly,  it  has  long  been  re- 
marked by  the  fishermen  of  the  Lipari  Islands  that  in  stormy 
weather  there  is  at  Stromboli  a  more  copious  discharge  of 
steam  and  stones  than  in  fine  weather.  They  make  use  of 
the  cone  as  a  weather-glass,  the  increase  of  its  activity  indi- 
cating a  falling,  and  the  diminution  a  rising  barometer.  In 
like  manner,  Etna,  according  to  Sartorius  von  Waltershau- 
sen,  is  most  active  in  the  winter  months.  Mr.  Coan  has  in- 
dicated a  relation  between  the  eruptions  of  Kilauea  and  the 
rainy  seasons  of  Hawaii,  most  of  the  discharges  of  that 
crater  taking  place  within  the  four  months  from  March  to 
June.35 

When  we  remember  the  connection,  now  indubitably 
established,  between  a  more  copious  discharge  of  fire-damp 
in  mines  and  a  lowering  of  atmospheric  pressure,  we  may  be 
prepared  to  find  a  similar  influence  affecting  the  escape  of 
vapors  from  the  upper  surface  of  the  lava-column  of  a  vol- 
cano; for  it  is  not  so  much  to  the  lava  itself  as  to  the  expan- 
sive vapors  impregnating  it  that  the  manifestations  of  vol- 
canic activity  are  due.  Among  the  Vesuvian  eruptions  since 


35  Dana,  "Characteristics  of  Volcanoes,"  p.  125.  For  accounts  of  the  vol- 
canic phenomena  of  Hawaii,  see  W.  Ellis,  "Polynesian  Researches. "  Wilkes' 
TT.  S.  Exploring  Expedition,  1838-42,  "Geology,"  by  J.  D.  Dana.  The  Rev. 
T.  Coan,  a  missionary  resident  in  Hawaii,  observed  the  operations  of  the  vol- 
canoes for  upward  of  forty  years,  and  published  from  time  to  time  short  notices 
of  them  in  the  American  Journal  of  Science,  vols.  xiii.  (1852)  xiv.,  xv.,  xviii., 
xxi.,  xxii.,  xxiii.,  xxv.,  xxvii.,  xxxvii.,  xl.,  xliii.,  xlvii.,  xlix;  3d  ser.  ii.  (1871) 
iv.,  vii.,  viii.,  xiv.,  xviii.,  xx.,  xxi.,  xxii.  (1881).  Prof.  Dana  has  recently  re- 
visited these  volcanoes  and  fully  discussed  their  phenomena  in  the  Amer.  Jouru. 
Sci.  vols.  xxxiii. -xxxvii.  (1887-89),  and  in  his  "Characteristics  of  Volcanoes." 
See  also  C.  E.  Dutton,  Amer.  Journ.  Sci.  xxv.  (1883),  p.  219;  Report  U.  S. 
Geological  Survey,  1882-83.  L.  Green,  "Vestiges  of  the  Molten  Globe,"  1887. 
For  an  account  of  the  remarkable  glassy  lavas  of  Hawaii,  see  E.  Cohen,  Neues 
Jahrb.  1880  (ii.),  p.  23;  and  a  general  account  of  the  petrography  of  the  islands, 
by  E.  S.  Dana,  Amer.  Journ.  Sci.  xxxvii.  (1889),  p.  441. 


DYNAMICAL    GEOLOGY  851 

the  middle  of  the  seventeenth  century,  the  number  which 
took  place  in  winter  and  spring  has  been  to  that  of  those 
which  broke  out  in  summer  and  autumn  as  7  to  4.  In 
Japan,  also,  the  greater  number  of  recorded  eruptions  have 
taken  place  during  the  cold  months  of  the  year,  February 
to  April.36 

There  may  be  other  causes  besides  atmospheric  pressure 
concerned  in  these  differences;  the  preponderance  of  rain 
during  the  winter  and  spring  may  be  one  of  these.  Accord- 
ing to  Mr.  Coan,  previous  to  the  great  Hawaiian  eruption  of 
1868  there  had  been  unusually  wet  weather,  and  to  this  fact 
he  attributes  the  exceptional  severity  of  the  earthquakes  and 
volcanic  explosions.  The  greater  frequency  of  Japanese 
volcanic  eruptions  and  earthquakes  in  winter  has  been  re- 
ferred in  explanation  to  the  fact  that  the  average  barometric 
gradient  across  Japan  is  steeper  in  winter  than  in  summer, 
while  the  piling  up  of  snow  in  the  northern  regions  gives 
rise  to  long-continued  stresses,  in  consequence  of  which  cer- 
tain lines  of  weakness  in  the  earth's  crust  are  more  prepared 
to  give  way  during  the  winter  months  than  they  are  in  sum- 
mer." The  effects  of  varying  atmospheric  pressure,  how- 
ever, can  probably  only  slightly  and  locally  modify  volcanic 
activity.  Eruptions,  like  the  great  one  of  Cotopaxi  in  1877, 
have  in  innumerable  instances  taken  place  without,  so  far  as 
can  be  ascertained,  any  reference  to  atmospheric  conditions. 

Kluge  has  sought  to  trace  a  connection  between  the  years 
of  maximum  and  minimum  sun-spots  and  those  of  greatest 
and  feeblest  volcanic  activity,  and  has  constructed  lists  to 
show  that  years  which  have  been  specially  characterized  by 
terrestrial  eruptions  have  coincided  with  those  marked  by 

36  J.  Milne,  Seismol.  Soc.  Japan,  IX.  Part  ii.  p.  174. 
*>  J.  Milne,  loc.  cit 


352  TEXT-BOOK   OF   GEOLOGY 

few  sun-spots  and  diminished  magnetic  disturbance.88  Such 
a  connection  cannot  be  regarded  as  having  yet  been  satisfac- 
torily established.  Again,  the  same  author  has  called  atten- 
tion to  the  frequency  and  vigor  of  volcanic  explosions  at  or 
near  the  time  of  the  August  meteoric  shower.  But  in  this 
case,  likewise,  the  cited  examples  can  hardly  yet  be  looked 
upon  as  more  than  coincidences. 

Periodicity  of  Eruptions.— At  many  volcanic  vents  the 
eruptive  energy  manifests  itself  with  more  or  less  regularity. 
At  Stromboli,  which  is  constantly  in  an  active  state,  the  ex- 
plosions occur  at  intervals  varying  from  three  or  four  to  ten 
minutes  and  upward.  A  similar  rhythmical  movement  has 
been  often  observed  during  the  eruptions  at  other  vents 
which  are  not  constantly  active.  Volcano,  for  example, 
during  its  eruption  of  September,  1873,  displayed  a  succes- 
sion of  explosions  which  followed  each  other  at  intervals  of 
from  twenty  to  thirty  minutes.  At  Etna  and  Vesuvius  a  simi- 
lar rhythmical  series  of  convulsive  efforts  has  often  been  ob- 
served during  the  course  of  an  eruption."  Among  the  vol- 
canoes of  the  Andes  a  periodic  discharge  of  steam  has  been 
observed:  Mr.  Whymper  noticed  outrushes  of  steam  to  pro- 
ceed at  intervals  of  from  twenty  to  thirty  minutes  from  the 
summit  of  Sangai,  while  during  his  inspection  of  the  great 
crater  of  Cotopaxi,  this  volcano  was  seen  to  blow  off  steam 
at  intervals  of  about  half  an  hour.40  At  the  eruption  of  the 
Japanese  volcano,  Oshima,  in  1877,  Mr.  Milne  observed  that 
the  explosions  occurred  nearly  every  two  seconds,  with  occa- 


1  "Ueber  Synchronismus  und  Antagonismus,"  8vx>,  Leipzig,  1863,  p.  72. 
A.  Poey  (Comptes  Rend.  Ixxviii.  (1874),  p.  51)  believes  that  among  the  786  erup- 
tions recorded,  by  Kluge,  between  1749  and  1861,  the  maxima  correspond  to 
periods  of  minimum  in  solar  spots.  See,  however,  postea,  p.  477 

39  G.  Mercalli,  Atti.  Soc.  Ital.  Sci.  Nat.  xxiv.  (1881). 

40  "Travels  amongst  the  Great  Andes  of  the  Equator,"  1892,  pp.  74,  163. 


DYNAMICAL    GEOLOGY  353 

sional  pauses  of  15  or  20  seconds.41  Kilauea,  in  Hawaii, 
seems  to  show  a  regular  system  of  grand  eruptive  periods. 
Dana  has  pointed  out  that  outbreaks  of  lava  have  taken 
place  from  that  volcano  at  intervals  of  from  eight  to  nine 
years,  this  being  the  time  required  to  fill  the  crater  up  to  the 
point  of  outbreak,  or  to  a  depth  of  400  or  500  feet." 

Some  volcanoes  have  exhibited  a  remarkable  paroxysmal 
phase  of  activity,  when  after  comparative  or  complete  quies- 
cence a  sudden  gigantic  explosion  has  taken  place,  followed 
by  renewed  and  prolonged  repose.  Vesuvius  supplies  the 
most  familiar  illustration  of  this  character  of  volcanic  en- 
ergy. The  great  eruption  of  A.D.  79,  which  truncated  the 
upper  part  of  the  old  cone  of  Somma,  was  a  true  paroxysmal 
explosion,  unlike  anything  that  had  preceded  it  within  his- 
toric times,  and  far  more  violent  than  any  subsequent  mani- 
festation of  the  same  volcano.  The  crater-basin  of  Santorin, 
of  which  the  islands  Thera  and  Therasia  represent  portions 
of  the  rim,  seems  to  have  been  blown  out  by  some  stupen- 
dous paroxysm  in  prehistoric  times.  The  vast  explosion  of 
Krakatoa  in  1883  was  another  memorable  example.  In 
these  instances  there  was  an  earlier  period  of  ordinary  vol- 
canic activity,  during  which  a  large  cone  was  gradually  built 
up.  In  the  case  of  Somma  and  Krakatoa  the  energy  died 
down  for  a  time,  and  the  paroxysm  came  with  hardly  any 
premonitory  warning.  It  has  been  succeeded  by  a  time  of 
comparatively  feeble  activity.  At  Vesuvius  the  great  ex- 
plosion of  1631,  which  terminated  nearly  1500  years  of 
quiescence,  may  be  regarded  as  a  minor  paroxysm,  since 
which  the  mountain  has  remained  more  continuously  active. 


41  Trans.  Seism.  Soc.  Japan,  ix.  part  ii.  p.  82. 

4-2  "Characteristics  of  Volcanoes,"  p.  124.     On  periodicity  of  eruptions,  see 
Kluge,  Neues  Jahrb.  1862,  p.  582. 


854  TEXT-BOOK    OF   GEOLOGY 

General  sequence  of  events  in  an  Eruption. — The  ap- 
proach of  an  eruption  is  not  always  indicated  by  any  pre- 
monitory symptoms,  for  many  tremendous  explosions  are 
recorded  to  have  taken  place  in  different  parts  of  the  world 
without  perceptible  warning.  Much  in  this  respect  would 
appear  to  depend  upon  the  condition  of  liquidity  of  the  lava, 
and  the  amount  of  resistance  offered  by  it  to  the  passage 
of  the  escaping  vapors  through  its  mass.  In  Hawaii,  where 
the  lavas  are  remarkably  liquid,  vast  outpourings  of  them 
have  taken  place  quietly  without  earthquakes  during  the 
present  century.  But  even  there,  the  great  eruption  of 
1868  was  accompanied  by  violent  earthquakes. 

The  eruptions  of  Vesuvius  are  often  preceded  by  failure 
or  diminution  of  wells  and  springs.  But  more  frequent  in- 
dications of  an  approaching  outburst  are  conveyed  by  sym- 
pathetic movements  of  the  ground.  Subterranean  rumblings 
and  groanings  are  heard;  slight  tremors  succeed,  increasing 
in  frequency  and  violence  till  they  become  distinct  earth- 
quake shocks.  The  vapors  from  the  crater  grow  more  abun- 
dant, as  the  lava-column  in  the  pipe  or  funnel  of  the  volcano 
ascends,  forced  upward  and  kept  in  perpetual  agitation  by 
the  passage  of  elastic  vapors  through  its  mass.  After  a 
long  previous  interval  of  quiescence,  there  may  be  much 
solidified  lava  toward  the  top  of  the  funnel,  which  will  re- 
strain the  ascent  of  the  still  molten  portion  underneath.  A 
vast  pressure  is  thus  exercised  on  the  sides  of  the  cone, 
which,  if  too  weak  to  resist,  will  open  in  one  or  more  rents, 
and  the  liquid  lava  will  issue  from  the  outer  slope  of  the 
mountain;  or  the  energies  of  the  volcano  will  be  directed 
toward  clearing  the  obstruction  in  the  chief  throat,  until  with 
tremendous  explosions,  and  the  rise  of  a  vast  cloud  of  dust 
and  fragments,  the  bottom  and  sides  of  the  crater  are  finally 


DYNAMICAL    GEOLOGY  355 

blown  out,  and  the  top  of  the  cone  disappears.  The  lava 
ma}'  now  escape  from  the  lowest  part  of  the  lip  of  the  crater, 
while,  at  the  same  time,  immense  numbers  of  red-hot  bombs, 
scoriae,  and  stones  are  shot  up  into  the  air.  The  lava  at  first 
rushes  down  like  one  or  more  rivers  of  melted  iron,  but,  as 
it  cools,  its  rate  of  motion  lessens.  Clouds  of  steam  rise 
from  its  surface,  as  well  as  from  the  central  crater.  Indeed, 
every  successive  paroxysmal  convulsion  of  the  mountain  is 
marked,  even  at  a  distance,  by  the  rise  of  huge  ball-like 
wreaths  or  clouds  of  steam,  mixed  with  dust  and  stones, 
forming  a  column  which  towers  sometimes  a  couple  of  miles 
or  more  above  the  summit  of  the  cone.  By  degrees  these 
eructations  diminish  in  frequency  and  intensity.  The  lava 
ceases  to  issue,  the  showers  of  stones  and  dust  decrease,  and 
after  a  time,  which  may  vary  from  hours  to  days  or  months, 
even  in  the  regime  of  the  same  mountain,  the  volcano  be- 
comes once  more  tranquil.4* 

In  the  investigation  of  the  subject,  the  student  will  nat- 
urally devote  attention  specially  to  those  aspects  of  volcanic 
action  which  have  more  particular  geological  interest  from 
the  permanent  changes  with  which  they  are  connected,  or 
from  the  way  in  which  they  enable  us  to  detect  and  realize 
conditions  of  volcanic  energy  in  former  periods. 

Fissures. — The  convulsions  which  culminate  in  the  forma- 
tion of  a  volcano  usually  split  open  the  terrestrial  crust  by 
a  more  or  less  nearly  rectilinear  fissure,  or  by  a  system  Of 
fissures.  In  the  subsequent  progress  of  the  mountain,  the 
ground  at  and  around  the  focus  of  action  is  liable  to  be  again 
and  again  rent  open  by  other  fissures.  These  tend  to  di- 


43  See  Schmidt's  narrative  of  the  eruption  of  Vesuvius  in  May,  1855  (ante, 
p.  333).  An  account  of  the  great  eruption  of  Cotopaxi  in  June,  1877,  by  Dr. 
Th.  Wolf,  will  be  found  in  Neues  Jahrb.  1878,  p.  113. 


356  TEXT-BOOK    OF   GEOLOGY 

verge  from  the  focus;  but  around  the  vent  where  the  rocks 
have  been  most  exposed  to  concussion,  the  fissures  some- 
times intersect  each  other  in  all  directions.  In  the  great 
eruption  of  Etna,  in  the  year  1669,  a  series  of  six  parallel 
fissures  opened  on  the  side  of  the  mountain.  One  of  these, 
with  a  width  of  two  yards,  ran  for  a  distance  of  12  miles,  in 
a  somewhat  winding  course,  to  within  a  mile  of  the  top  of 
the  cone.44  Similar  fissures,  but  on  a  smaller  scale,  have 
often  been  observed  on  Vesuvius  and  other  volcanoes.**  A 
fissure  sometimes  reopens  for  a  subsequent  eruption. 

Two  obvious  causes  may  be  assigned  for  the  pushing 
upward  of  a  crater-floor  and  the  fissuring  of  a  volcanic 
cone — (1)  the  enormous  pressure  of  the  dissolved  vapors 
or  gases  acting  upon  the  walls  and  roof  of  the  funnel  and 
convulsing  the  cone  by  successive  explosions;  and  (2)  the 
hydrostatic  pressure  of  the  lava-column  in  the  funnel,  which 
may  be  taken  to  be  about  120  Ibs.  per  square  inch,  or  nearly 
8  tons  on  the  square  foot,  for  each  100  feet  of  depth.  Both 
of  these  causes  may  act  simultaneously,  and  their  united 
effect  has  been  to  uplift  enormous  superincumbent  masses 
of  solid  rock  and  to  produce  a  widespread  series  of  long 
and  continuous  fissures  reaching  from  unknown  depths  to 
various  distances  from  the  surface  and  even  opening  up 
sometimes  on  the  surface.  These  results  of  the  expansive 
energy  of  volcanic  action  are  of  special  interest  to  the 
geologist,  for  he  encounters  evidence  of  similar  operations 
in  former  times  preserved  in  the  crust  of  the  earth  (see 
Book  IV.  Part  VII.  Sect.  i.). 

Into  rents   thus    formed,   the  water-substance  or  vapor 


44  For  fissures  on  Etna,  see  Silvestri,  Boll.  R.  Geol.  Com.  Ital.  1874. 

45  For   a  description  of  those  of  Iceland  (which  run  chiefly  N.E.  to  S.W., 
and  N.  to  S.)  see  T.  Kjerulf,  Nyt.  Mag.  xxi.  147. 


DYNAMICAL    GEOLOGY 


357 


rises  with  great  expansive  force,  followed  by  the  lava 
which  solidifies  there  like  iron  in  a  mold.  Where  fissures 
are  vertical  or  highly  inclined  the  igneous  rock  takes  the 
form  of  dikes  or  veins]  where  the  intruded  material  has 
forced  its  way  more  or  less  in  a  horizontal  direction  be- 
tween strata  of  tuff,  beds  of  non-volcanic  sediments,  or 


Fjg.  42.— View  of  Lava-dikes,  Valle  del  Bove,  Etna  (Abich). 

flows  of  lava,  it  takes  the  form  of  sheets  (sills)  or  beds.  The 
cliffs  of  many  an  old  crater  show  how  marvellously  they 
have  been  injected  by  such  veins,  dikes,  or  sheets  of  lava. 
Those  of  Somma,  and  the  Valle  del  Bove  on  Etna  (Fig.  42), 
which  have  long  been  known,  project  now  from  the  softer 
tuffs  like  walls  of  masonry.48  The  crater  cliffs  of  Santorin 
also  present  an  abundant  series  of  dikes.  The  permanent 
separation  of  the  walls  of  fissures  by  the  consolidation  of 
the  lava  that  rises  in  them  as  dikes  must  widen  the  dimen- 

46  S.  von  Waltershausen  "Der  Aetna,"  ii.  p.  341. 


358 


TEXT-BOOK    OF   GEOLOGY 


sions  of  a  cone,  for  the  fissures  are  not  due  to  shrinkage, 
although  doubtless  the  loosely  piled  fragmentary  materials, 
in  the  course  of  their  consolidation,  develop  lines  of  joint. 
Sometimes  the  lava  has  evidently  risen  in  a  state  of  extreme 
fluidity,  and  has  at  once  filled  the  rents  prepared  for  it, 


Fig.  43.— Dike  contorting  beds  of  tuff.    Crater  of  Vesuvius  (Abich). 

cooling  rapidly  on  the  outside  as  a  true  volcanic  glass,  but 
assuming  a  distinctly  crystalline  structure  inside  (ante,  p. 
296).  Dikes  of  this  kind,  with  a  vitreous  crust  on  their 
sides,  may  be  seen  on  the  crater-wall  of  Somma,  and  not 
uncommonly  among  basalt  dikes  in  Iceland  and  Scotland. 
In  other  cases,  the  lava  had  probably  already  acquired 
a  more  viscous  or  even  lithoid  char- 
acter ere  it  rose  in  the  fissure,  and 
in  this  condition  was  able  to  push 
aside  and  even  contort  the  strata  of 
tuff  through  which  it  made  its  way 

Fig.  44.— Section    of   Dikes   of     ,_..         .  _,.         „..  ,       ,.     ,       ,       , 

Lava  traversing  the  bedded   (Fig.  43).     There  can  be  little  doubt 

tuffs  of  a  volcanic  cone. 

that  in  the  architecture  of  a  volcano, 

dikes  must  act  the  part  of  huge  beams  and  girders  (Fig. 
44),  binding  the  loose  tuffs  and  intercalated  lavas  together, 
and  strengthening  the  cone  against  the  effects  of  subse- 
quent convulsions. 


DYNAMICAL    GEOLOGY  B59 

From  this  point  of  view,  an  explanation  suggests  itself 
of  the  observed  alternations  in  the  character  of  a  volcano's 
eruptions.  These  alternations  may  depend  in  great  measure 
upon  the  relation  between  the  height  of  the  cone,  on  the 
one  hand,  and  the  strength  of  its  sides,  on  the  other.  When 
the  sides  have  been  well  braced  together  by  interlacing 
dikes,  and  further  thickened  by  the  spread  of  volcanic 
materials  all  over  their  slopes,  they  may  resist  the  effects 
of  explosion  and  of  the  pressure  of  the  ascending  lava- 
column.  In  this  case,  the  volcano  may  find  relief  only 
from  its  summit,  and  if  the  lava  flows  forth,  it  will  do  so 
from  the  top  of  the  cone.  As  the  cone  increases  in  eleva- 
tion, however,  the  pressure  from  within  upon  its  sides  aug- 
ments. Eventually  egress  is  once  more  established  on  the 
flanks  by  means  of  fissures,  and  a  new  series  of  lava-streams 
is  poured  out  over  the  lower  slopes  (see  Fig.  62). 

In  the  deeper  portions  of  a  volcanic  vent  the  convulsive 
efforts  of  the  lava-column  to  force  its  way  upward  must 
often  produce  lateral  as  well  as  vertical  rifts,  aud  into 
these  the  molten  material  will  rush,  exerting  as  it  goes  an 
enormous  upward  pressure  on  the  mass  of  rock  overlying 
it.  At  a  modern  volcano  these  subterranean  manifestations 
cannot  be  seen,  but  among  the  volcanoes  of  Tertiary  and 
older  time  they  have  been  revealed  by  the  progress  of 
denudation.  Some  of  these  older  examples  teach  us  the 
prodigious  upheaving  power  of  the  sheets  of  molten  rock 
intruded  between  volcanic  or  other  strata.  An  account  of 
this  structure  (sills,  laccolites),  with  reference  to  some 
examples  of  it,  will  be  found  in  Book  IV.  Part  VII.*T 

Though    lava    very  commonly    issues   from   the   lateral 

«  See  particularly  the  description  of  intrusive  sheets  or  laccolites. 


360  TEXT-BOOK    OF    GEOLOGY 

fissures  on  a  volcanic  cone,  it  may  sometimes  approach  the 
surface  in  thejn  without  actually  flowing  out.  The  great 
fissure  on  E'tna  in  1669,  for  example,  was  visible  even  from 
a  distance,  by  the  long  line  of  vivid  light  which  rose  from 
the  incandescent  lava  within.  Again,  it  frequently  happens 
that  minor  volcanic  cones  are  thrown  up  on  the  line  of  a 
fissure,  either  from  the  congelation  of  the  lava  round  the 
point  of  emission,  or  from  the  accumulation  of  ejected 
scoriae  round  the  fissure-vent.  One  of  the  most  remark- 
able examples  of  this  kind  is  that  of  the  Laki  fissure  in 
Iceland,  the  whole  length  of  which  (12  miles)  bristles  with 
small  cones  and  craters  almost  touching  each  other." 

Explosions. — Apart  from  the  appearance  of  visible  fis- 
sures, volcanic  energy  may  be,  as  it  were,  concentrated 
on  a  given  point,  which  will  usually  be  the  weakest  in  the 
structure  of  that  part  of  the  terrestrial  crust,  and  from 
which  the  solid  rock,  shattered  ,into  pieces,  is  hurled  into 
the  air  by  the  enormous  expansive  energy  of  the  volcanic 
vapors."  This  operation  has  often  been  observed  in  vol- 
canoes already  formed,  and  has  even  been  witnessed  on 
ground  previously  unoccupied  by  a  volcanic  vent.  The 
history  of  the  cone  of  Vesuvius  brings  before  us  a  long 
series  of  such  explosions,  beginning  with  that  of  A.D.  79, 
and  coming  down  to  the  present  day  (Fig.  45).  Even  now, 
in  spite  of  all  the  lava  and  ashes  poured  out  during  the  last 
eighteen  centuries,  it  is  easy  to  see  how  stupendous  must 
have  been  that  earliest  explosion,  by  which  the  southern 
half  of  the  ancient  crater  was  blown  out.  At  every  suc- 

48  A.  Eelland,  "Lakis  Kratere  og  Lava-strome, "  cited  on  p.  345.     On  this 
straight  fissure  some  600  craters  rise,  varying  from  6  to  450  feet  high. 

49  See  Daubree's  experiments  on  the  mechanical  effects  of  gas  at  high  pres- 
sures, Comptes  Rend,  cxi.,  cxii.    cxiii.  and  Bull.  Soc.  Geol.  France,  xix.  (1891), 
p.  313. 


DYNAMICAL    GEOLOGY  361 

cessive  important  eruption,  a  similar  but  minor  operation 
takes  place  within  the  present  cone.  The  hardened  cake 
of  lava  forming  the  floor  is  burst  open,  and  with  it  there 
usually  disappears  much  of  the  upper  part  of  the  cone,  and 
sometimes,  as  in  1872,  a  large  segment  of  the  crater-wall. 
The  Valle  del  Bove  on  the  eastern  flank  of  Etna  is  a  chasm 
probably  due  mainly  to  some  gigantic  prehistoric  explo- 
sion.80 The  islands  of  Santorin  (Figs.  65  and  66)  bring 
before  us  evidence  of  a  prehistoric  catastrophe  of  a  similar 


?.  45.— View  of  Vesuvius  from  the  south, 
Showing  the  remaining  part  of  the  old  crater-wall  of  Somma  behind. 

nature,  by  which  a  large  volcanic  cone  was  blown  up.  The 
existing  outer  islands  are  a  chain  of  fragments  of  the  pe- 
riphery of  the  cone,  the  centre  of  which  is  now  occupied  by 
the  sea.  In  the  year  1538  a  new  volcano,  Monte  Nuovo, 
was  formed  in  24  hours  on  the  margin  of  the  Bay  of  Naples. 
An  opening  was  drilled  by  successive  explosions,  and  such 
quantities  of  stones,  scori*,  and  ashes  were  thrown  out  from 
it  as  to  form  a  hill  that  rose  440  English  feet  above  the  sea- 
level,  and  was  more  than  a  mile  and  a  half  in  circumfer- 
ence. Most  of  the  fragments  c^w  to  be  seen  on  the  slopes 
of  this  cone  and  inside  its  beautifully  perfect  crater  are  of 

60  "Der  Aetna,"  p.  400. 
GEOLOGY— Vol.  XXIX— 16 


362  TEXT-BOOK    OF   GEOLOGY 

various  volcanic  rocks,  many  of  them  being  black  scoriae; 
but  pieces  of  Roman  pottery,  together  with  fragments  of 
the  older  underlying  tuff,  and  some  marine  shells,  have 
been  obtained — doubtless  part  of  the  soil  and  subsoil  dis- 
located and  ejected  during  the  explosions. 

One  of  the  most  stupendous  volcanic  explosions  on 
record  was  that  of  Krakatoa  in  the  Sunda  Strait  on  the 
26th  and  27th  of  August,  1883. 61  After  a  series  of  convul- 
sions, the  greater  portion  of  the  island  was  blown  out  with 
a  succession  of  terrific  detonations  which  were  heard  more 
than  150  miles  away.  A  mass  of  matter,  estimated  at  about 
1-g  cubic  miles  in  bulk,  was  hurled  into  the  air  in  the  form 
of  lapilli,  ashes,  and  the  finest  volcanic  dust.  The  effects 
of  this  volcanic  outburst  were  marked  both  upon  the  atmos- 
phere and  the  ocean.  A  series  of  barometrical  disturbances 
passed  round  the  globe  in  opposite  directions  from  the  vol- 
cano at  the  rate  of  about  700  miles  an  hour.  The  air-wave, 
travelling  from  east  to  west,  is  supposed  to  have  passed  three 
and  a  quarter  times  round  the  earth  (or  82,200  miles)  before 
it  ceased  to  be  perceptible."  The  sea  in  the  neighborhood 
was  thrown  into  waves,  one  of  which  was  computed  to  have 
risen  more  than  100  feet  above  tide-level,  destroying  towns, 
villages,  and  36,380  people.  Oscillations  of  the  water  were 
perceptible  even  at  Aden,  1000  miles  distant,  at  Port  Eliza- 
beth in  South  Africa,  5450  miles,  and  among  the  islands  of 
the  Pacific  Ocean,  and  they  are  computed  to  have  travelled 
with  a  maximum  velocity  of  467  statute  miles  in  the  hour." 


51  See  "The  Eruption  of  Krakatoa,"  by  a  Committee  of  the  Royal  Society, 
1888.  "Krakatau,"  R.  D.  M.  Verbeck,  Batavia,  1886. 

58  Scott  and  Strachey,  Proc.  Roy.  Soc.  xxxvi.  (1883).  Royal  Society's 
Report,  p.  57. 

63  Wharton,  Royal  Society's  Report,  p.  89.  For  a  great  Japanese  explosion, 
see  Nature,  13th  Sept.  1888. 


DYNAMICAL    GEOLOGY  363 

It  is  not  necessary,  and  it  does  not  always  happen,  that 
any  actual  solid  or  liquid  volcanic  rock  is  erupted  by  explo- 
sions that  shatter  the  rocks  through  which  the  funnel  passes. 
Thus,  among  the  cones  of  the  extinct  volcanic  tract  of  the 
Eifel,  some  occur  which  consist  entirely,  or  nearly  so,  of 
comminuted  debris  of  the  surrounding  Devonian  graywacke 
and  slate  through  which  the  various  volcanic  vents  have 
been  opened  (see  pp.  341,  417,  970).  Evidently,  in  such 
cases,  only  elastic  vapors  forced  their  way  to  the  surface; 
and  we  see  what  probably  often  takes  place  in  the  early 
stages  of  a  volcano's  history,  though  the  fragments  of  the 
underlying  disrupted  rocks  are  in  most  instances  buried  and 
lost  under  the  far  more  abundant  subsequent  volcanic  mate- 
rials. Sections  of  small  ancient  volcanic  "necks"  or  pipes 
sometimes  afford  an  excellent  opportunity  of  observing  that 
these  orifices  were  originally  opened  by  the  blowing  out  of 
the  solid  crust  and  not  by  the  formation  of  fissures.  Exam- 
ples will  be  cited  in  later  pages  from  Scottish  volcanic  areas 
of  Old  Red  Sandstone,  Carboniferous,  and  Permian  age. 
The  orifices  are  there  filled  with  fragmentary  materials, 
wherein  portions  of  the  surrounding  and  underlying  rocks 
form  a  noticeable  proportion.*4  (See  Figs.  296-301.) 

Showers  of  Dost  and  Stones.— A  communication  having 
been  opened,  either  by  fissuring  or  explosion,  between  the 
heated  interior  and  the  surface,  fragmentary  materials  are 
commonly  ejected  from  it,  consisting  at  first  mainly  of 'the 
rocks  through  which  the  orifice  has  been  opened,  afterward 
of  volcanic  substances.  In  a  great  eruption,  vast  numbers 
of  red-hot  stones  are  shot  up  into  the  air,  and  fall  back 
partly  into  the  crater  and  partly  on  the  outer  slopes  of  the 

54  Trans.  Roy.  Soc.  Edin.  xxix.  p.  458;  Quart.  Journ.  Geol.  Soc.  (1892), 
President's  Address,  pp.  86,  118,  135,  143,  153. 


864  TEXT-BOOK    OF   GEOLOGY 

cone.  According  to  Sir  W.  Hamilton,  cinders  were  thrown 
by  Vesuvius,  during  the  eruption  of  1779,  to  a  height  of 
10,000  feet.  Instances  are  known  where  large  stones,  ejected 
obliquely,  have  described  huge  parabolic  curves  in  the  air, 
and  fallen  at  a  great  distance.  Stones  8  Ibs.  in  weight  occur 
among  the  ashes  which  buried  Pompeii.  The  volcano  of 
Antuco  in  Chile  is  said  to  send  stones  flying  to  a  distance 
of  36  (?)  miles,  Cotopaxi  is  reported  to  have  hurled  a  200- 
ton  block  9  miles,66  and  the  Japanese  volcano,  Asama,  is 
said  to  have  ejected  many  blocks  of  stone,  measuring  from 
40  to  more  than  100  feet  in  diameter.66 

But  in  many  great  eruptions,  besides  a  constant  shower 
of  stones  and  scoriae,  a  vast  column  of  exceedingly  fine  dust 
rises  out  of  the  crater,  sometimes  to  a  height  of  several 
miles,  and  then  spreads  outward  like  a  sheet  of  cloud.  The 
remarkable  fineness  of  this  dust  may  be  understood  from 
the  fact  that  during  great  volcanic  explosions  no  boxes, 
watches,  or  close-fitting  joints  have  been  found  to  be  able 
to  exclude  it.  Mr.  Whymper  collected  some  dust  that  fell 
65  miles  away  from  Cotopaxi,  and  which  was  so  fine  that 
from  4,000  to  25,000  particles  were  required  to  weigh  a 
grain."  So  dense  is  the  dust-cloud  as  to  obscure  the  sun, 
and  for  days  together  the  darkness  of  night  may  reign  for 
miles  around  the  volcano.  In  1822,  at  Vesuvius,  the  ashes 
not  only  fell  thickly  on  the  villages  round  the  base  of  the 
mountain,  but  travelled  as  far  as  Ascoli,  which  is  56  Italian 
miles  distant  from  the  volcano  on  one  side,  and  as  Casano, 
105  miles  on  the  other.  The  eruption  of  Cotopaxi,  on  26th 

55  D.  Forbes,  Geol.  Mag.  vii.  p.  320. 

56  J.  Milne,  Seism.  Soc.  Japan,  ix.  p.  179,  where  an  excellent  account  of  the 
volcanoes  of  Japan  is  given.     See  also  "The  Volcanoes  of  Japan,"  by  J.  Milne 
and  W.  K.  Burton. 

M  Royal  Society  Report  on  Krakatoa,  p.  183. 


DYNAMICAL    GEOLOGY  365 

June,  1877,  began  by  an  explosion  that  sent  up  a  column  of 
fine  ashes  to  a  prodigious  height  into  the  air,  where  it  rapidly 
spread  out  and  formed  so  dense  a  canopy  as  to  throw  the 
region  below  it  into  total  darkness.68  So  quickly  did  it 
diffuse  itself,  that  in  an  hour  and  a  half,  a  previously  bright 
morning  became  at  Quito,  33  miles  distant,  a  dim  twilight, 
which  in  the  afternoon  passed  into  such  darkness  that  the 
hand  placed  before  the  eye  could  not  be  seen.  At  Guaya- 
quil, on  the  coast,  150  miles  distant,  the  shower  of  ashes 
continued  till  the  1st  of  July.  Dr.  Wolf  collected  the  ashes 
daily,  and  estimated  that  at  that  place  there  fell  315  kilo- 
grammes on  every  square  kilometre  during  the  first  thirty 
hours,  and  on  the  30th  of  June,  209  kilogrammes  in  twelve 
hours.68  During  a  much  less  important  eruption  of  the  same 
mountain  on  3d  July,  1880,  the  amount  of  volcanic  dust 
ejected,  according  to  Mr.  Whymper,  could  not  have  been 
less,  and  was  probably  vastly  more,  than  two  millions  of 
tons,*0  equivalent  to  a  mass  of  lava  containing  more  than 
150,000  cubic  feet. 

The  explosion  of  Krakatoa  in  August,  1883,  was  accom- 
panied by  the  discharge  of  enormous  quantities  of  volcanic 
dust,  some  of  which  was  carried  to  vast  distances.  It  was 
estimated  that  the  clouds  of  fine  dust  were  hurled  from  that 
volcano  to  a  height  of  17  miles,  and  the  darkness  which  they 
caused  extended  for  150  miles  from  the  focus  of  eruption. 
The  diffusion  and  continued  suspension  of  the  finer  particles 


58  During  the  comparatively  insignificant  eruption  of  the  volcano  in  1880 
Mr.  Whymper  noticed  that  a  column  of  inky  blackness,  formed  doubtless  of 
volcanic  dust,  went  straight  up  into  the  air  with  such  velocity  that  in  less  than  a 
minute  it  had  risen  20,000  feet  above  the  rim  of  the  crater,  or  40,000  feet  above 
the  sea.     "Travels  amongst  the  Great  Andes,"  p.  322. 

59  Neues  Jahrb.  1878,  p.  141.     An  account  of  this  eruption  is  given  by  Mr. 
Whymper  in  his  "Travels  amongst  the  Great  Andes,"  chap.  vi. 

eo  "Travels  amongst  the  Great  Andes,"  p.  328. 


366  TEXT-BOOK    OF    GEOLOGY 

of  this  dust  in  the  upper  air  has  been  regarded  as  the  proba- 
ble cause  of  the  remarkably  brilliant  sunsets  of  the  follow- 
ing winter  and  spring  over  a  large  part  of  the  earth's  sur- 
face.61 One  of  the  most  stupendous  outpourings  of  volcanic 
ashes  on  record  took  place,  after  a  quiescence  of  26  years, 
from  the  volcano  Coseguina,  in  Nicaragua,  during  the  early 
part  of  the  year  1835.  On  that  occasion,  utter  darkness  pre- 
vailed over  a  circle  of  35  miles  radius,  the  ashes  falling  so 
thickly  that,  even  8  leagues  from  the  mountain,  they  cov- 
ered the  ground  to  a  depth  of  about  10  feet.  It  was  esti- 
mated that  the  rain  of  dust  and  sand  fell  over  an  area  at 
least  270  geographical  miles  in  diameter.  Some  of  the  finer 
materials,  thrown  so  high  as  to  come  within  the  influence  of 
an  upper  air-current,  were  borne  away  eastward,  and  fell, 
four  days  afterward,  at  Kingston,  in  Jamaica — a  distance  of 
700  miles.  During  the  great  eruption  of  Sumbawa,  in  1815, 
the  dust  and  stones  fell  over  an  area  of  nearly  one  million 
square  miles,  and  were  estimated  by  Zollinger  to  amount  to 
fully  fifty  cubic  miles  of  material,  and  by  Junghuhn  to  be 
equal  to  one  hundred  and  eighty-five  mountains  like  Vesu- 
vius. Toward  the  end  of  the  18th  century,  during  a  time 
of  great  disturbance  among  the  Japanese  volcanoes,  one  of 
them,  Sakurajima,  threw  out  so  much  pumiceous  material 
that  it  was  possible  to  walk  a  distance  of  23  miles  upon  the 
floating  debris  in  the  sea. 

An  inquiry  into  the  origin  of  these  showers  of  fragmen- 
tary materials  brings  vividly  before  us  some  of  the  essential 
features  of  volcanic  action.  We  find  that  bombs,  slags,  and 
lapilli  may  be  thrown  up  in  comparatively  tranquil  states  of 
a  volcano,  but  that  the  showers  of  fine  dust  are  discharged 

«  Royal  Society  Report,  pp.  151-463. 


DYNAMICAL    GEOLOGY  367 

with  violence,  and  only  appear  when  the  volcano  becomes 
more  energetic.  Thus,  at  the  constantly,  but  quietly,  active 
volcano  of  Stromboli,  the  column  of  lava  in  the  pipe  may  be 
watched  rising  and  falling  with  a  slow  rhythmical  move- 
ment. At  every  rise,  the  surface  of  the  lava  swells  up  into 
blisters  several  feet  in  diameter,  which  by  and  by  burst  with 
a  sharp  explosion  that  makes  the  walls  of  the  crater  vibrate. 
A  cloud  of  steam  rushes  out,  carrying  with  it  hundreds  of 
fragments  of  the  glowing  lava,  sometimes  to  a  height  of  1200 
feet.  It  is  by  the  ascent  of  steam  through  its  mass,  that  a 
column  of  lava  is  kept  boiling  at  the  bottom  of  the  crater, 
and  by  the  explosion  of  successive  larger  bubbles  of  steam, 
that  the  various  bombs,  slags,  and  fragments  of  lava  are  torn 
off  and  tossed  into  the  air.  It  has  often  been  noticed  at 
Vesuvius  that  each  great  concussion  is  accompanied  by  a 
huge  ball-like  cloud  of  steam  which  rushes  up  from  the 
crater.  Doubtless  it  is  the  sudden  escape  of  that  steam 
which  causes  the  explosion. 

The  varying  degree  of  liquidity  or  viscosity  of  the  lava 
probably  modifies  the  force  of  explosions,  owing  to  the  dif- 
ferent amounts  of  resistance  offered  to  the  upward  passage 
of  the  absorbed  gases  and  vapors.  Thus  explosions  and  ac- 
companying scoriae  are  abundant  at  Vesuvius,  where  the 
lavas  are  comparatively  viscid;  they  are  almost  unknown  at 
Kilauea,  where  the  lava  is  remarkably  liquid. 

In  tranquil' conditions  of  a  volcano,  the  steam,  whether 
collecting  into  larger  or  smaller  vesicles,  works  its  way  up- 
ward through  the  substance  of  the  molten  lava,  and  as  the 
elasticity  of  this  compressed  vapor  overcomes  the  pressure 
of  the  overlying  lava,  it  escapes  at  the  surface,  and  there  the 
lava  is  thus  kept  in  ebullition.  But  this  comparatively 
quiet  operation,  which  may  be  watched  within  the  craters 


368  TEXT-BOOK    OF   GEOLOGY 

of  many  active  volcanoes,  does  not  produce  clouds  of  fine 
dust.  The  collision  or  friction  of  millions  of  stones  ascend- 
ing and  descending  in  the  dark  column  above  the  crater 
must  doubtless  cause  much  dust  and  sand.  But  the  explo- 
sive action  of  steam  is  probably  also  an  immediate  cause  of 
much  trituration.  The  aqueous  vapor  or  water-gas  which 
is  so  largely  dissolved  in  many  lavas  must  exist  within  the 
lava-column,  under  an  enormous  pressure,  at  a  temperature 
far  above  its  critical  point  (p.  332),  even  at  a  white  heat,  and 
therefore  possibly  in  a  state  of  dissociation.  The  sudden 
ascent  of  lava  so  constituted  relieves  the  pressure  rapidly 
without  sensibly  affecting  the  temperature  of  the  mass. 
Consequently,  the  white-hot  gases  or  vapors  at  length  ex- 
plode, and  reduce  the  molten  mass  to  the  finest  powder,  like 
water  shot  out  of  a  gun." 

Evidently  no  part  of  the  operations  of  a  volcano  has 
greater  geological  significance  than  the  ejection  of  such 
enormous  quantities  of  fragmentary  matter.  In  the  first 
place,  the  fall  of  these  loose  materials  round  the  orifice  of 
discharge  is  one  main  cause  of  the  growth  of  the  volcanic 
cone.  The  heavier  fragments  gather  around  the  vent,  and 
there  too  the  thickest  accumulation  of  dust  and  sand  takes 
place.  Hence,  though  successive  explosions  may  blow  out 
the  upper  part  of  the  crater- walls  and  prevent  the  mountain 
from  growing  so  rapidly  in  height,  every  eruption  must  in- 
crease the  diameter  of  the  cone.  In  the  second  place,  as 
every  shower  of  dust  and  sand  adds  to  the  height  of  the 
ground  on  which  it  falls,  thick  volcanic  accumulations  may 


68  Messrs.  Murray  and  Eenard  (Proc.  Roy.  Soc.  Bdin.  xii.  (1884),  p.  480) 
concluded  that  the  fragmentary  condition  and  the  fresh  fractures  of  the  dust 
particles  of  the  Krakatoa  eruption  were  due  to  a  tension  phenomenon,  which 
affects  these  vitreous  matters  in  a  manner  analogous  to  what  is  observed  in 
"Rupert's  drops." 


DYNAMICAL    GEOLOGY  Sbi) 

be  formed  far  beyond  the  base  of  the  mountain.  The  vol- 
cano of  Sangay,  in  Ecuador,  for  instance,  has  buried  the 
country  around  it  to  a  depth  of  4000  feet  under  its  ashes." 
In  such  loose  deposits  are  entombed  trees  and  other  kinds  of 
vegetation,  together  with  the  bodies  of  animals,  as  well  as 
the  works  of  man.  In  some  cases,  where  the  layer  of  vol- 
canic dust  is  thin,  it  may  merely  add  to  the  height  of  the 
soil,  without  sensibly  interfering  with  the  vegetation.  But 
it  has  been  observed  at  Santorin  that  though  this  is  true  in 
dry  weather,  the  fall  of  rain  with  the  dust  at  once  acts  detri- 
mentally. On  the  3d  of  June,  1866,  the  vines  were  there 
withered  up,  as  if  they  had  been  burned,  along  the  track  of 
the  smoke  cloud.*4  By  the  gradual  accumulation  of  vol- 
canic ashes,  new  geological  formations  arise  which,  in  their 
component  materials,  not  only  bear  witness  to  the  volcanic 
eruptions  that  produced  them,  but  preserve  a  record  of  the 
land-surfaces  over  which  they  spread.  In  the  third  place, 
besides  the  distance  to  which  the  fragments  may  be  hurled 
by  volcanic  explosions,  or  to  which  they  may  be  diffused  by 
the  ordinary  aerial  movements,  we  have  to  take  into  account 
the  vast  spaces  across  which  the  liner  dust  is  sometimes 
borne  by  upper  air-currents.  In  the  instance  already  cited, 
ashes  from  Coseguina  fell  700  miles  away,  having  been  car- 
ried all  that  long  distance  by  a  high  counter-current  of  air, 
moving  apparently  at  the  rate  of  about  seven  miles  an  hour 
in  an  opposite  direction  to  that  of  the  wind  which  blew  at  the 
surface.  By  the  Sumbawa  eruption,  also  referred  to  above, 
the  sea  west  of  Sumatra  was  covered  with  a  layer  of  ashes 
two  feet  thick.  On  several  occasions  ashes  from  the  Ice- 
landic volcanoes  have  fallen  so  thickly  between  the  Orkney 

63  D.  Forbes,  Geol.  Mag.  vii.  320.  **  Fouque,  "Santorin,"  p.  81. 


370  TEXT-BOOK    OF   GEOLOGY 

and  Shetland  Islands,  that  vessels  passing  there  have  had 
the  unwonted  deposit  shovelled  off  their  decks  in  the  morn- 
ing. In  the  year  1783,  during  the  memorable  eruption  of 
Skaptar-Jdkull,  so  vast  an  amount  of  fine  dust  was  ejected 
that  the  atmosphere  over  Iceland  continued  loaded  with  it 
for  months  afterward.  It  fell  in  such  quantities  over  parts 
of  Caithness — a  distance  of  600  miles — as  to  destroy  the 
crops;  that  year  is  still  spoken  of  by  the  inhabitants  as  the 
year  of  "the  ashie."  Traces  of  the  same  deposit  have  been 
observed  in  Norway,  and  even  as  far  as  Holland.'6  Hence 
it  is  evident  that  volcanic  accumulations  may  take  place  in 
regions  many  hundreds  of  miles  distant  from  any  active  vol- 
cano. A  single  thin  layer  of  volcanic  detritus  in  a  group  of 
sedimentary  strata  would  not  thus  of  itself  prove  the  exist- 
ence of  contemporaneous  volcanic  action  in  its  neighbor- 
hood. Failing  other  proof  of  adjacent  volcanic  activity,  it 
might  have  been  wind-borne  from  a  volcano  in  a  distant 
region. 

Lava-streams. — At  its  exit  from  the  side  of  a  volcano, 
lava  glows  with  a  white  heat,  and  flows  with  a  motion  which 
has  been  compared  to  that  of  honey  or  of  melted  iron.  It 
soon  becomes  red,  and  like  a  coal  fallen  from  a  hot  fireplace, 
rapidly  grows  dull  as  it  moves  along,  until  it  assumes  a 
black,  cindery  aspect.  At  the  same  time  the  surface  con- 
geals, and  soon  becomes  solid  enough  to  support  a  heavy 
block  of  stone.  The  aspect  of  the  stream  varies  with  the 
composition  and  fluidity  of  the  lava,  form  of  the  ground, 
angle  of  slope,  and  rapidity  of  flow.  Viscous  lavas,  like 
those  of  Vesuvius,  break  up  along  the  surface  into  rough 


66  Nordenskiold,  Geol.  Mag.  2d  dec.  iii.  p.  292.  G.  vom  Rath,  Mouatsber. 
K.  Preuss.  Akad.  Wiss.  1876,  p.  282.  Neues  Jahrb.  1876,  p.  52,  and  postea, 
p.  675. 


DYNAMICAL    GEOLOGY 


371 


brown  or  black  cinder-like  slags  and  irregular  ragged  cakes, 
bristling  with  jagged  points  ("aa"  86),  which,  in  their  onward 
motion,  grind  and  grate  against  each  other  with  a  harsh  me- 
tallic sound,  sometimes  rising  into  rugged  mounds  or  be- 
coming seamed  with  rents  and  gashes,  at  the  bottom  of 
which  the  red-hot  glowing  lava  may  be  seen  (Fig.  46).  In 
lavas  possessing  somewhat  greater  fluidity,  the  surface  pre- 
sents froth-like,  curving  lines,  as  in  the  scum  of  a  slowly 


Figr.  46.— View  of  portion  of  a  Lava-stream  on  Vesuvius  (Abich). 

flowing  river,  or  is  arranged  in  curious  ropy  folds,  as  the 
layers  nave  successively  flowed  over  each  other  and  con- 
gealed ("pahoehoe"  "").  These,  and  many  other  fantastic 
coiled  shapes  were  exhibited  by  the  Yesuvian  lava  of  1858, 
and  are  admirably  displayed  by  the  peculiarly  liquid  glassy 
lavas  of  Kilauea.86  Basalts  possessing  extreme  liquidity 

66  For  descriptions  of  Vesuvian  lava-streams,  see  the  various  memoirs  and 
works  cited,  anle,  p.  333.  For  those  of  Etna,  Sartorius  von  Waltershausen  and 
A.  von  Lasaulx,  "Der  Aetna,"  ii.  p.  390.  The  rugged  scoriaceous  lava-surfaces 
are  known  in  Hawaii  as  aa,  the  smooth  coiled  and  ropy  surfaces  are  there  called 


TEXT-BOOK    OF    GEOLOGY 

have  flowed  for  great  distances  with  singularly  smooth  sur- 
faces. A  large  area  which  has  been  flooded  with  lava  is 
perhaps  the  most  hideous  and  appalling  scene  of  desolation 
anywhere  to  be  found  on  the  surface  of  the  globe. 

A  lava-stream  usually  spreads  out  as  it  descends  from  its 
point  of  escape,  and  moves  more  slowly.  Its  sides  look  like 
huge  embankments,  or  like  some  of  the  long  mounds  of 


Pig.  47.— View  of  houses  surrounded  and  partly  demolished  by  the  Lava 
of  Vesuvius,  1872. 

"clinkers"  in  a  great  manufacturing  district.  The  advancing 
end  is  often  much  steeper,  creeping  onward  like  a  great  wall 
or  rampart,  down  the  face  of  which  the  rough  blocks  of 
hardened  lava  are  ever  rattling  (Fig.  47). 

Outflow  of  Lava. — This  appears  to  be  immediately 
due  to  the  expansion  of  the  absorbed  vapors  and  gases  in 
the  molten  rock.  Though  these  vapors  may  reach  the  sur- 


paJwehoe.  Dana,  "Characteristics  of  Volcanoes,"  p.  9.  The  same  stream  of 
lava  may  exhibit  both  these  aspects  in  different  parts  of  its  course.  Ibid.  p.  209 
and  Mr.  Johnston-Lavis'  papers  on  Vesuvius,  already  cited  p.  333. 


DYNAMICAL    GEOLOGY  373 

face,  and  even  produce  tremendous  explosions,  without  an 
actual  outcome  of  lava,  yet  so  intimately  are  vapors  and 
lava  commingled  in  the  subterranean  reservoirs,  that  they 
commonly  rise  together,  and  the  explosions  of  the  one  lead 
to  the  outflow  of  the  other.  The  first  point  at  which  the 
lava  makes  its  appearance  at  the  surface  will  largely  depend 
upon  the  structure  of  the  ground.  Two  causes  have  been 
assigned  on  a  foregoing  page  (p.  356)  for  the  fissuring  of  a 
volcanic  cone.  As  the  molten  mass  rises  within  the  chim- 
ney of  the  volcano,  continued  explosions  of  vapor  take 
place  from  its  upper  surface.  The  violence  of  these  may 
be  inferred  from  the  vast  clouds  of  steam,  ashes,  and  stones 
hurled  to  so  great  a  height  into  the  air,  and  from  the  con- 
cussions of  the  ground,  which  may  be  felt  at  distances  of 
more  than  100  miles  from  the  volcano.  It  need  not  be 
a  matter  of  surprise,  therefore,  that  the  sides  of  a  great 
vent,  exposed  to  shocks  of  such  intensity,  should  at  last 
give  way,  and  that  large  divergent  fissures  should  be 
opened  down  the  cone.  Again,  the  hydrostatic  pressure 
of  the  column  of  lava  must,  at  a  depth  of  1000  feet  below 
the  top  of  the  column,  exert  a  pressure  of  between  70  and 
80  tons  on  each  square  foot  of  the  surrounding  walls  (p.  356). 
We  may  well  believe  that  such  a  force,  acting  upon  the 
walls  of  a  funnel  already  shattered  by  a  succession  of  ter- 
rific explosions,  may  prove  too  great  for  their  resistance. 
When  this  happens,  the  lava  pours  forth  from  the  outside 
of  the  cone.  On  a  much-fissured  cone,  lava  may  issue 
freely  from  many  points,  so  that  a  volcano  so  affected  has 
been  graphically  described  as  "sweating  fire." 

In  a  lofty  volcano,  lava  occasionally  rises  to  the  lip  of 
the  crater  and  flows  out  there;  but  more  frequently  it 
escapes  from  some  fissure  or  orifice  in  a  weak  part  of  the 


874  TEXT-BOOK   OF   GEOLOGY 

cone.  In  minor  volcanoes,  on  the  other  hand,  where  the 
explosions  are  less  violent,  and  where  the  thickness  of 
the  cone  in  proportion  to  the  diameter  of  the  funnel  is  often 
greater,  the  lava  very  commonly  rises  into  the  crater. 
Should  the  crater-walls  be  too  weak  to  resist  the  pressure 
of  the  molten  mass,  they  give  way,  and  the  lava  rushes 
out  from  the  breach.  This  is  seen  to  have  happened  in 
several  of  the  puys  of  Auvergne,  so  well  figured  and  de- 
scribed by  Scrope  (Fig.  48). "  But  if  the  crater  be  massive 


Fig.  48.— View  of  one  of  the  Tuff-cones  of  Auvergne,  broken  aown  on  one  side 
by  the  escape  of  a  stream  of  Lava.    (After  Scrope.) 

enough  to  withstand  the  pressure,  the  lava  may  at  last  flow 
out  from  the  lowest  part  of  the  rim. 

In  a  tall  column  of  molten  lava,  there  may  be  a  varia- 
tion in  the  density  of  its  different  parts,  the  heaviest 
naturally  gravitating  to  the  bottom.  It  has  been  observed 
by  Ch.  Velain  that  at  the  Isle  of  Bourbon  (Reunion),  the 
lavas  escaping  from  the  base  of  the  volcanic  cone  are  denser 


61  For  descriptions  of  this  region,  see  Scrope's  "Geology  and  Extinct  Volca- 
noes of  Central  France, "  2d  edit.  1858.  H.  Lecoq's  "Epoques  ge"ologiques  de 
1'Auvergne,"  1867.  Michel-Levy,  Bull.  Soc.  Geol.  France,  xviii.  (1890),  p.  688. 
The  succession  of  volcanic  rocks  in  Velay  is  described  by  M.  Boule,  Bull.  Soc. 
Geol.  France,  xviii.  (1889),  p.  174,  and  in  Bull.  Carte  Geol.  de  la  France,  No.  28 
(1892);  see  also  op.  cit.  No.  13  for  a  memoir  by  P.  Termier. 


DYNAMICAL    GEOLOGY  375 

and  more  basic  than  those  which  flow  out  from  the  lip  of 
the  crater.88 

As  soon  as  the  molten  rock  reaches  the  surface,  the 
superheated  water- vapor  or  gas,  dissolved  within  its  mass, 
escapes  copiously,  and  hangs  as  a  dense  white  cloud  over 
the  moving  current.  The  lava-streams  of  Vesuvius  some- 
times appear  with  as  dense  a  steam-cloud  at  their  lower 
ends  as  that  which  escapes  at  the  same  time  from  the  main 
crater.  Even  after  the  molten  mass  has  flowed  several 
miles,  steam  continues  to  rise  abundantly  both  from  its 
end  and  from  numerous  points  along  its  surface,  and  con- 
tinues to  do  so  for  many  weeks,  months,  or  it  may  be  for 
several  years. 

Should  the  point  of  escape  of  a  lava-stream  lie  well 
down  on  the  cone,  far  below  the  summit  of  the  lava-column 
in  the  funnel,  the  molten  rock,  on  its  first  escape,  driven 
by  hydrostatic  pressure,  will  sometimes  spout  up  high  into 
the  air — a  fountain  of  molten  rock.  This  was  observed  in 
1794  on  Vesuvius,  and  in  1832  on  Etna.  In  the  eruption 
of  1852  at  Mauna  Loa,  an  unbroken  fountain  of  lava,  from 
200  to  700  feet  in  height  and  1000  feet  broad,  burst  out  at 
the  base  of  the  cone.  Similar  "geysers"  of  molten  rock 
have  subsequently  been  noticed  in  the  same  region.  Thus 
in  March  and  April,  1868,  four  fiery  fountains,  throwing 
lava  to  heights  varying  from  500  to  1000  feet,  continued 
to  play  for  several  weeks.  According  to  Mr.  Coan,  such 
outbursts  take  place  from  the  bottom  of  a  column  of  lava 
3000  feet  high.  The  volcano  of  Mauna  Loa  strikingly  illus- 
trates another  feature  of  volcanic  dynamics  in  the  position 
and  outflow  of  lava.  It  bears  upon  its  flanks  at  a  distance 

68  l'Les  Volcans,"  p.  36.     For  references  relating  to  this  island,  see  p.  416. 


376  TEXT-BOOK    OF    GEOLOGY 

of  20  miles,  but  10,000  feet  lower,  the  huge  crater  Kilauea. 
As  Dana  has  pointed  out,  these  orifices  form  part  of  one 
mountain,  yet  the  column  of  lava  stands  10,000  feet  higher 
in  one  conduit  than  in  the  other.  On  a  far  smaller  scale 
the  same  independence  occurs  among  the  several  pipes  of 
some  of  the  geysers  in  the  Yellowstone  region  of  North 
America. 

From  the  wide  extent  of  basalt-dikes,  such  as  those  of 
Tertiary  age  in  Britain,  which  rise  to  the  surface  at  a  dis- 
tance of  200  miles  from  the  main  remnants  of  the  volcanic 
outbursts  of  their  time,  and  are  found  over  an  area  of  per- 
haps 100,000  square  miles,  it  is  evident  that  molten  lava 
may  sometimes  occupy  a  far  greater  space  within  the  crust 
than  might  be  inferred  from  the  dimensions  and  outpour- 
ings even  of  the  largest  volcanic  cone.  There  can  be  no 
doubt  that  vast  reservoirs  of  melted  rock,  impregnated  with 
superheated  vapors,  must  formerly  have  existed,  if  they  do 
not  exist  still,  beneath  extensive  tracts  of  countiy  (p.  967). 
Yet  even  in  these  more  stupendous  manifestations  of  vol- 
canism,  the  lava  should  be  regarded  rather  as  the  sign  than 
as  the  cause  of  volcanic  action.  The  cause  of  the  ascent  of 
the  lava  in  volcanic  pipes  is  still  obscure:  it  may  possibly 
be  due  to  the  compression  arising  from  the  secular  contrac- 
tion of  the  earth.  But  it  is  doubtless  the  pressure  of  the 
imprisoned  vapor,  and  its  struggles  to  get  free,  which  pro- 
duce the  subterranean  earthquakes  and  the  explosions  from 
the  vents.  As  soon  as  the  vapor  finds  relief,  the  terrestrial 
commotion  calms  down  again,  until  another  accumulation 
of  vapor  demands  a  repetition  of  the  same  phenomena. 

Bate  of  flow  of  Lava. — The  rate  of  movement  is 
regulated  by  the  fluidity  of  the  lava,  by  its  volume,  and 
by  the  form  and  inclination  of  the  ground.  Hence,  as  a 


DYNAMICAL    GEOLOGY  377 

rule,  a  lava-stream  moves  faster  at  first  than  afterward,  be- 
cause it  has  not  had  time  to  stiffen,  and  its  slope  of  descent 
is  usually  steeper  than  further  down  the  mountain.  One 
of  the  most  fluid  and  swiftly  flowing  lava-streams  ever 
observed  on  Vesuvius  was  that  erupted  on  12th  August, 
1805.  It  is  said  to  have  rushed  down  a  space  of  3  Italian 
(3f  English)  miles  in  the  first  four  minutes,  but  to  have 
widened  out  and  moved  more  slowly  as  it  descended,  yet 
finally  to  have  reached  Torre  del  Greco  in  three  hours.  A 
lava  erupted  by  Maun  a  Loa  in  1852  went  as  fast  as  an  ordi- 
nary stage-coach,  or  fifteen  miles  in  two  hours;  but  some  of 
the  lavas  from  that  mountain  have  in  parts  of  their  course 
moved  with  double  that  rapidity.  Long  after  a  current 
has  been  deeply  crusted  over  with  slags  and  rough  slabs 
of  lava,  it  may  continue  to  creep  slowly  forward  for  weeks 
or  even  months. 

It  happens  sometimes  that,  as  the  lava  moves  along,  the 
still  molten  mass  inside  bursts  through  the  outer  hardened 
and  deeply  seamed  crust,  and  rushes  out  with,  at  first,  a 
motion  much  more  rapid  than  that  of  the  main  stream. 
Any  sudden  change  in  the  form  or  slope  of  the  ground 
affects  the  flow  of  the  lava.  Thus,  reaching  the  edge  of 
a  steep  defile  or  cliff,  the  molten  rock  pours  over  in  a  cata- 
ract of  glowing,  molten  rock,  with  clouds  of  steam,  showers 
of  fragments,  and  a  noise  utterly  indescribable.  Or,  on  the 
other  hand,  encountering  a  ridge  or  hill  across  its  path,  it 
accumulates  until  it  either  finds  egress  round  the  side  or 
actually  overrides  and  entombs  the  obstacle.  The  hardened 
crust  or  shell,  within  which  the  still  fluid  lava  moves,  serves 
to  keep  the  mass  from  spreading.  Here  and  there,  inside 
this  crust,  the  lava  subsides,  leaving  cavernous  spaces  and 
tunnels  into  which,  when  the  whole  is  cold,  one  may  creep, 


378  TEXT-BOOK    OF   GEOLOGY 

and  which  are  sometimes  festooned  with  stalactites  of  lava 
(p.  387). 

Size  of  Lava-streams. — In  some  cases,  lava  escap- 
ing from  craters  or  fissures  comes  to  rest  before  reaching 
the  base  of  the  slopes,  like  the  obsidian  current  which  has 
congealed  on  the  side  of  the  little  volcanic  island  of  Vol- 
cano.8' In  other  instances,  the  molten  rock  not  only  reaches 
the  plains  but  flows  for  many  miles  away  from  the  point  of 
eruption.  Sartorius  von  Waltershausen  computed  the  lava 
emitted  by  Etna  in  1865  at  92  millions  of  cubic  metres,  that 
of  1852  at  420  millions,  that  of  1669  at  980  millions,  and 
that  of  a  prehistoric  lava-stream  near  Randazzo  at  more 
than  1000  millions.70  The  most  stupendous  outpouring  of 
lava  on  record  was  that  which  took  place  in  Iceland  in  the 
year  1783.  Successive  streams  issued  from  a  fissure  about 
12  miles  long,  filling  up  river-gorges  which  were  sometimes 
600  feet  deep  and  200  feet  broad,  and  advancing  into  the 
alluvial  plains  in  lakes  of  molten  rock  12  to  15  miles  wide 
and  100  feet  deep.  Two  currents  of  lava  which,  filling  up 
the  valley  of  the  Skapta,  escaped  in  nearly  opposite  direc- 
tions, extended  for  45  and  50  miles  respectively,  their  usual 
thickness  being  100  feet.  Bischof  estimated  that  the  total 
amount  of  lava  poured  forth  during  this  single  eruption 
"surpassed  in  magnitude  the  bulk  of  Mont  Blanc."71 

Varying  liquidity  of  Lava. — All  lava,  at  the 
time  of  its  expulsion,  is  in  a  molten  condition.  It  usually 


69  Recent  eruptions  in  this  island  have  consisted  entirely  of  ashes.  A.  Balt- 
zer,  Zeitsch.  Deutsch.  Geol.  Ges.  xxvi.  (1875),  p.  36.  G.  Mercalli,  "Le  Eruzi- 
oni  dell'  Isola  Vulcano, "  Rassegna  Nazionale,  1889;  also  a  paper  by  same 
author  in  Atti.  Soc.  Ital.  Sci.  Nat.,  vol.  xxxi. 

10  "Der  Aetna,"  ii.  303. 

11  Lyell,   "Principles,"  ii.  p.  49.      Holland,  "Lakis  Kratere,"  cited  ante, 
p.  345. 


DYNAMICAL    GEOLOGY  379 

consists  of  a  glassy  magma  in  which,  by  reason  of  the  high 
temperature,  most  or  even  all  of  the  mineral  constituents 
exist  dissolved.  Considerable  differences,  however,  have 
been  observed  in  the  degree  of  liquidity.  Humboldt  and 
Scrope  long  ago  called  attention  to  the  thick,  short,  lumpy 
form*  presented  by  masses  of  solidified  trachytic  rocks, 
which  are  lighter  and  more  siliceous,  and  to  the  thin, 
widely  extended  sheets  assumed  by  basalts,  which  are 
heavy  and  contain  much  iron  and  basic  silicates.™  It  may 
be  inferred  that,  as  a  rule,  the  basalts  or  basic  lavas  have 
been  more  liquid  than  the  trachytes  or  siliceous  lavas. 
The  cause  of  this  difference  has  been  variously  explained. 
It  may  depend  partly  upon  chemical  composition,  the  sili- 
ceous being  naturally  less  fusible  than  the  basic  rocks. 
But  as  great  differences  of  fluidity  are  observable  even 
among  lavas  having  nearly  the  same  composition,  there 
would  seem  to  be  some  further  cause  for  the  diversity. 
Eeyer  has  ingeniously  maintained  that  we  must  look  to 
original  differences  in  the  extent  to  which  the  subterra- 
nean igneous  magma  that  supplied  the  lava  has  been 
saturated  with  vapors  and  gases.  Molten  rock  highly  im- 
pregnated gives  rise,  he  holds,  to  fragmentary  discharges, 
while  when  feebly  impregnated  it  flows  out  tranquilly." 
On  the  other  hand,  Captain  C.  E.  Dutton,  who  has  studied 
the  volcanic  phenomena  of  Western  America  and  Hawaii, 
suggests  that  the  different  degrees  of  liquidity  may  depend 
not  only  on  chemical  differences,  but  on  variations  of  tem- 
perature. He  supposes  that  the  basaltic  lavas  which  have 
spread  so  far  in  thin  sheets,  and  which  must  have  had  a 
comparatively  great  liquidity,  flowed  at  temperatures  far 

TO  Scrope,  "Considerations  on  Volcanoes"  (1825),  p.  93. 
13  "Beitrag  zur  Physik  der  Eruptionen,"  p.  77. 


380  TEXT-BOOK   OF   GEOLOGY 

above  that  of  their  melting-point,  and  were,  to  use  his 
phrase,  "superfused."74 

The  varying  degrees  of  liquidity  are  manifested  in  a 
characteristic  way  on  the  surface  of  lava.  Thus,  in  the 
great  lava-pools  of  Hawaii,  the  rock  exhibits  a  remarkable 
liquidity,  throwing  up  fountains  of  molten  rock  to  a  height 
of  300  feet  or  more.  During  its  ebullition  in  the  crater- 
pools,  jets  and  dribblets,  a  quarter 
of  an  inch  in  diameter,  are  tossed 
up,  and  falling  back  on  one  another, 
make  "a  column  of  hardened  tears 
of  lava,"  one  of  which  (Fig.  49) 
was  found  to  have  attained  a  height 

*       &j>'f., ~';'iv^vX  of  40  feet,  while  in  other  places  the 

K^-Z$~  •  •  •!•'•'•' :  H;  >v01&v^ 

•JwSsbhiL^L-^^JSc^sSPI    jets  thrown  up  and  blown  aside  by 

6f  Kilauea  (Dana)/'     glagg  which  He  thickly  together   like 

mown  grass,  and  are  known  by  the  natives  under  the  name 
of  "Pele's  Hair,"  after  one  of  their  divinities."  Yet  al- 
though the  ebullition  is  caused  by  the  uprise  and  escape  of 
highly  heated  vapors,  there  is  no  cloud  over  the  boiling  lake 
itself,  heavy  white  vapor  only  escaping  at  different  points 
along  the  edge. 

On  the  other  hand,  the  lavas  of  Vesuvius  and  of  most 
modern  volcanoes,  which  issue  so  saturated  with  vapor  as  to 
be  nearly  concealed  from  view  in  a  cloud  of  steam,  are  ac- 
companied by  abundant  explosions  of  fragmentary  materials. 
Slags  and  clinkers,  torn  by  explosions  of  steam  from  the 


14  "High  Plateaus  of  Utah,"  Geog.  and  Geol.  Sur.  Territories.  Washing- 
ton, 1880,  chap.  v. 

16  Dana,  Geol.  U.  S.  Explor.  Exped.,  "Geology,"  p.  179;  "Characteristics 
of  Volcanoes,"  p.  160. 


DYNAMICAL    GEOLOGY 


881 


molten  rock,  are  strewn  abundantly  over  the  cone,  while  the 
surface  of  the  lava  is  likewise  rugged  with  similar  clinkers, 
which  may  now  and  then  be  observed  piled  up  round  some 
more  energetic  steam-spiracle.  Sometimes  the  vapor  forces 
up  the  lava  round  such  a  spiracle  or  furnarole  and  gradually 
piles  up  a  rugged  column  several  feet  or  yards  in  height,  as 


Fig.  50.— Lava-column  (eight  feet  high),  Vesuvius  (Abich). 

has  been  observed  on  Vesuvius78  (Figs.  46,  49,  50).  So  vast 
an  amount  of  steam  rushes  out  from  one  of  these  orifices, 
and  with  such  boiling  and  explosion,  that  the  cone  of  bombs, 
slags,  and  irregular  lumps  of  lava  forms  a  miniature  or  para- 
sitic volcano,  which  will  remain  as  a  marked  cone  on  its 

16  Some  good  examples  were  observed  on  this  mountain  in  the  summer  of 
1891  by  Mr.  Johnston -La  vis,  Brit.  Assoc.  1891,  sect.  C. 


382  TEXT-BOOK    OF   GEOLOGY 

parent  mountain  long  after  the  eruption  which  gave  it  birth 
has  ceased.  The  lava  of  the  eruption  at  Santorin  in  1866-67 
at  first  welled  out  tranquilly,  but  after  a  few  days  its  out- 
flow was  accompanied  by  explosions  and  discharges  of  in- 
candescent fragments,  which  increased  until  they  had  cov- 
ered the  lava  dome  with  ejected  scoria?,  and  had  opened  a 
number  of  crateriform  mouths  on  its  summit." 

There  can  be  no  doubt,  as  above  remarked,  that  the  con- 
dition of  liquidity  of  the  lava  has  in  some  measure  deter- 
mined the  form  of  the  eruptions.  In  one  case,  there  are 
quiet  outwellings  of  the  more  liquid  lavas,  as  at  Hawaii;  in 
another,  there  are  explosive  discharges  and  cinder-cones, 
accompanying  the  more  viscid  lavas,  as  at  most  modern  vol- 
canoes. The  former  has  been  the  condition  favorable  to  the 
most  colossal  outpourings  of  molten  rock,  as  we  see  in  the 
basalt-plateaus  of  Britain,  Faroe,  Greenland,  Idaho,  and 
Oregon,  the  Ghauts,  Abyssinia,  etc.  This  subject  is  again 
referred  to  at  p.  433. 

Crystallization  of  Lava. — Pouring  forth  with  a 
liquidity  like  that  of  molten  iron,  lava  speedily  assumes  a 
more  viscous  condition  and  a  slower  motion.  Obsidian  and 
other  vitreous  rocks  have  consolidated  as  glass:  yet  that 
they  are  not  always  extremely  fluid  is  indicated  by  the  arrest 
of  the  obsidian  stream  half-way  down  the  steep  northern 
slope  of  Volcano.  Even  in  such  perfect  natural  glass  as  ob- 
sidian, microscopic  crystallites  and  crystals  are  usually  pres- 
ent, and  in  prodigious  numbers  (pp.  205,  282).  In  most 
lavas,  devitrification  has  proceeded  so  far  before  the  final 
stiffening,  that  the  original  glassy  magma  has  passed  into  a 
more  or  less  completely  lithoid  or  crystalline  mass. 

17  Fouque,  "Santorin,"  p.  xv. 


DYNAMICAL    GEOLOGY  383 

That  lava  may  possess  an  appreciably  crystalline  struc- 
ture while  still  in  motion,  has  often  been  proved  at  Vesu- 
vius, where  well-defined  crystals  of  the  infusible  leucite  may 
be  observed  in  a  molten  magma  of  the  other  minerals,  por- 
tions of  the  white-hot  rock  in  this  condition  being  ladled 
out,  impressed  with  a  stamp  and  suddenly  congealed.  The 
fluxion-structure  above  described  (pp.  178,  213)  furnishes 
interesting  evidence  of  this  fact  in  many  ancient  as  well  as 
modern  lavas. 

There  is  reason  to  believe  that  in  the  molten  magma  be- 
neath a  volcano  considerable  progress  may  be  made  in  the 
development  of  some  crystalline  minerals  out  of  the  sur- 
rounding glass,  and  that  this  crystalline  portion  may  be  to 
some  extent  separated  from  the  vitreous  residue.  Hence 
where  this  has  taken  place,  subsequent  eruptions  may  give 
rise  to  a  more  crystalline  and  probably  more  basic  lava  from 
one  point  of  emission  and  a  more  glassy  and  probably  more 
acid  lava  from  another  vent.  Or  we  may  conceive  that  the 
two  portions  of  the  magma  may  be  subsequently  mingled 
again  in  various  proportions  before  eruption.78  If  the  proc- 
ess of  differentiation  should  continue,  as  seems  natural,  dur- 
ing the  lapse  of  a  whole  cycle  of  a  volcano's  history,  the 
earlier  lavas  would  be  more  basic  than  the  later. 

The  crystalline  structure  of  lava  has  been  supposed  to  be 
in  some  measure  determined  by  the  presence  of  the  volcanic 
vapors  and  gases  with  which  the  molten  rock  is  impregnated, 
the  rapid  escape  of  these  vapors  preventing  the  formation  of 
the  crystalline  structure,  and  leaving  the  lava  in  the  condi- 
tion of  a  more  or  less  perfect  glass.  But  the  experiments  of 


18  Compare  the  observation  of  Oh.  Velain  cited  ante,  p.  219,  and  the  remarks 
postea,  pp.  444,  457,  936.  Consult  on  this  subject  a  paper  by  Prof.  Judd,  Geol. 
Mag.  1888,  p.  1. 


384  TEXT-BOOK   OF   GEOLOGY 

MM.  Fouque*  and  Michel-LeVy  (postea,  p.  613)  have  shown 
that  rocks,  having  in  every  essential  particular  the  charac- 
ters of  volcanic  lavas,  may  be  artificially  produced  under 
ordinary  atmospheric  pressure  by  simple  dry  fusion.  There 
appears  to  be  no  doubt  that  the  presence  of  water  lowers  the 
fusion-point  of  silicates,  though  what  precise  influence  the 
dissolved  vapors  exert  upon  the  ultimate  consolidation  of 
molten  lava  has  yet  to  be  ascertained.  Difference  in  the  rate 
of  cooling  has  doubtless  been  an  important,  if  not  the  main, 
factor  in  determining  the  various  conditions  of  texture  of 
lava-streams.  The  crystalline  structure  may  be  expected  to 
be  most  perfect  where,  as  within  thick  masses  of  rock,  the 
cooling  has  been  prolonged,  and  where,  consequently,  the 
crystals  have  had  ample  time  and  opportunity  for  their  for- 
mation. On  the  other  hand,  the  glassy  structure  will  natu- 
rally be  most  perfectly  shown  where  the  cooling  has  been 
most  rapid,  as  in  the  vitreous  crust  on  the  walls  of  dikes  al- 
ready referred  to  (pp.  297,  358).  Rocks  crystallizing  in  the 
deeper  parts  of  a  volcano  usually  possess  a  more  coarsely 
crystalline  structure  than  those  which  crystallize  at  or  near 
to  the  surface  (p.  936). 

Temperature  of  Lav  a. — It  would  be  of  the  highest 
interest  and  importance  to  know  accurately  the  temperature 
at  which  a  lava-stream  first  issues.  Measurements  not  alto- 
gether satisfactory  have  been  taken  at  various  distances 
below  the  point  of  emission,  where  the  moving  lava  could 
be  safely  approached.  Experiments  made  at  Vesuvius  by 
Scacchi  and  Sainte-Claire  Deville  in  1855,  by  thrusting  thin 
wires  of  silver,  iron,  and  copper  into  the  lava,  indicated  a 
temperature  of  scarcely  700°  C.  (1228°  Fahr.).  Observations 
of  a  similar  kind,  made  in  1819,  when  a  silver  wire  3oth  inch 
in  diameter  at  once  melted  in  the  Vesuvian  lava  of  that  year, 


DYNAMICAL    GEOLOGY  385 

gave  a  greatly  higher  temperature,  the  melting-point  of  sil- 
ver being  about  1800°  Fahr.  But  copper  wire  has  also  been 
melted,  the  point  of  fusion  of  this  metal  being  about  2204° 
Fahr.  Evidence  of  the  high  temperature  of  lava  has  like- 
wise been  adduced  from  the  alteration  it  has  effected  upon 
refractory  substances  in  its  progress,  as  where,  at  Torre  del 
Greco,  it  overflowed  the  houses,  and  was  afterward  found  to 
have  fused  the  fine  edges  of  flints,  to  have  decomposed  brass 
into  its  component  metals,  the  copper  actually  crystallizing, 
and  to  have  melted  silver,  and  even  sublimed  it  into  small 
octahedral  crystals  (p.  393).  The  lava  of  Santorin  has  caught 
up  pieces  of  limestone,  and  has  formed  out  of  them  nodules 
containing  crystallized  anorthite,  augite,  sphene,  black  gar- 
net, and  particularly  wollastonite. "  The  initial  temperature 
of  lava,  as  it  first  issues  from  the  Yesuvian  funnel,  is  proba- 
bly considerably  more  than  2000°  Fahr.  Obviously  the  dis- 
solved water  (or  water-substance,  for,  as  already  remarked, 
the  temperature  is  far  above  the  critical  point  of  water,  and 
its  component  gases  may  exist  dissociated)  must  possess  as 
high  a  temperature  as  that  of  the  white-hot  lava  in  which 
it  is  contained.  The  existence  of  the  elements  of  water  at  a 
white  beat,  even  in  rocks  which  have  reached  the  surface,  is 
a  fact  of  no  little  significance  in  the  theoretical  consideration 
of  hypogene  action. 

Inclination  and  thickness  of  lava-flows. — 
It  was  at  one  time  supposed  that  lava  could  not  consoli- 
date in  beds  on  such  steep  slopes  as  those  of  most  volca- 
noes. Hence  arose  the  "elevation -crater  theory"  (described 
at  p.  412),  in  which  the  inclined  position  of  lavas  round 
a  volcanic  vent  was  explained  by  upheaval  after  their 


19  Fouque,  "Santorin,"  p.  206. 
GEOLOGY— Vol.  XXIX— 17 


386  TEXT-BOOK   OF   GEOLOGY 

emission.  Observations  all  over  the  world,  however,  have 
now  demonstrated  that  lava,  with  all  its  characteristic  fea- 
tures, can  consolidate  on  slopes  of  even  35°  and  40°. 80  The 
lava  in  the  Hawaii  Islands  has  cooled  rapidly  on  slopes  of 
25°,  that  from  Vesuvius,  in  1855,  is  here  and  there  as  steep 
as  30°,  while  the  older  lavas  in  Monte  Somma  are  sometimes 
inclined  at  45°.  On  the  east  side  of  Etna,  a  cascade  of  lava, 
which  in  1689  poured  into  the  vast  hollow  of  the  Cava 
Grande,  has  an  inclination  varying  from  18°  to  48°,  with 
an  average  thickness  of  16  feet.  On  Mauna  Loa  some  lava- 
flows  are  said  to  have  congealed  on  slopes  of  49°,  60°,  and 
even  90°, 81  though  in  these  cases  it  could  only  be  a  layer 
of  rock,  stiffening  and  adhering  to  the  surface  of  the  de- 
clivity. On  the  other  hand,  lava-streams  have  travelled 
considerable  distances  over  ground  that  to  the  eye  looks 
quite  level.  Among  the  Hawaiian  Islands  a  declivity  of 
1°  or  less  has  been  quite  sufficient  for  the  flow  of  the  ex- 
tremely liquid  and  mobile  lavas  of  that  region.  In  the 
great  lava-fields  of  the  Snake  River  region  of  the  Western 
Territories  of  the  United  States  the  basalts,  which  must 
also  have  been  extremely  liquid,  have  flowed  over  slopes 
of  much  less  than  I0.82  The  breadth  and  length  of  a  lava- 
stream,  as  well  as  the  form  of  its  surface,  depend  mainly 
upon  the  liquidity  of  the  molten  material  at  the  time  of 
outflow.  Even  when  it  consolidates  on  a  steep  slope,  a 
stream  of  lava  forms  a  sheet  with  parallel  upper  and  under 
surfaces,  a  general  uniformity  of  thickness,  and  often  greater 
evenness  of  surface,  than  where  the  angle  of  descent  is  low. 
The  thickness  varies  indefinitely;  many  basalts  which  have 


80  Lyell  on  the  consolidation  of  lava  on  steep  slopes,  Phil.  Trans.  1858. 
"'  J.  D.  Dana,  Amer.  Jour.  Sci.  xxxv.  (1888),  p.  32. 
82  J.  D.  Dana,  "Characteristics  of  Volcanoes,"  p.  12. 


DYNAMICAL    GEOLOGY  387 

been  poured  out  in  a  remarkably  liquid  condition  have 
solidified  in  beds  not  more  than  10  or  12  feet  thick.  On 
the  other  hand,  more  pasty  lavas,  and  lavas  which  have 
flowed  into  narrow  valleys,  may  be  piled  up  in  solid  masses 
to  a  thickness  of  several  hundred  feet  (pp.  378,  391). 

Structure  of  a  lava-stream. — Lava-streams  are 
sometimes  nearly  homogeneous  throughout.  In  general, 
however,  they  each  show  three  component  layers.  At  the 
bottom  lies  a  rough,  slaggy  mass,  produced  by  the  rapid 
cooling  of  the  lava,  and  the  breaking  up  and  continued 
onward  motion  of  the  scoriform  layer.  The  central  and 
main  portion  of  the  stream  consists  of  solid  lava,  often, 
however,  with  a  more  or 

less  carious   and   vesicular      e  -^^^^^^si:  J°^  fSS 
texture.      The  upper  part, 
as   we    have   seen,  may  be 
a   mass   of   rough    broken- 

UD    slabs,    SCOriae,    Or    clink-     Fig .  51.-Elongation  of  vesicles  in  direction 

of  flow  of  lava. 

ers.    The  proportions  borne 

by  these  respective  layers  to  each  other  vary  continu- 
ally. Some  of  the  more  fluid  ropy  lavas  of  Vesuvius 
have  an  inconstant  and  thin  slaggy  crust;  others  may  be 
said  to  consist  of  little  else  than  scoriae  from  top  to  bottom. 
Throughout  the  whole  mass  of  a  lava-current,  but  more 
especially  along  its  upper  surface,  the  absorbed  or  dis- 
solved water-vapor  expands  with  diminution  of  pressur-e, 
and  pushing  the  molten  rock  aside,  segregates  into  small 
bubbles  or  irregular  cavities.  Hence,  when  the  lava  solidi- 
fies, these  steam-holes  are  seen  to  be  sometimes  so  abundant 
that  a  detached  portion  of  the  rock  containing  them  will 
float  in  water  (pumice).  They  are  often  elongated  in  the 
direction  of  the  motion  of  the  lava-stream  (Fig.  51).  Some- 


388  TEXT-BOOK   OF   GEOLOGY 

times,  indeed,  where  the  cells  are  numerous,  their  elonga- 
tion in  one  direction  gives  a  fissile  structure  to  the  rock. 

A  singular  feature  in  many  lava-streams  are  the  tunnels 
and  caverns  already  referred  to  (p.  377)  as  observable  in 
them.  These  cavities  have  doubtless  arisen  during  the 
flow  of  the  mass  when  the  upper  and  under  portions  had 
solidified  and  were  creeping  sluggishly  onward,  while  the 
still  molten  interior  was  able  to  move  faster  and  thus  to 
leave  empty  spaces  behind  it.  Such  tunnels  may  frequently 
be  vseen  among  the  Vesuvian  lava-streams.  Some  remarkable 
examples  are  described  from  the  highly  glassy  lavas  of 
Hawaii,  where  they  are  sometimes  from  2  to  10  feet  in  height 
and  30  feet  broad,  but  with  large  lateral  expansions.  The 
walls  of  these  Hawaiian  lava-chambers  are  smooth  and  even 
glassy,  and  from  their  roofs  hang  slender  stalactites  of  lava 
20  to  30  inches  long,  while  on  the  floor  below  little  mounds 
of  lava-stalagmite  have  formed.  The  precise  mode  of  origin 
of  these  curious  appendages  is  not  yet  understood.83 

In  passing  from  a  fluid  to  a  solid  condition,  and  thus 
contracting,  lava  acquires  different  structures.  Lines  of 
divisional  planes  or  joints  traverse  it,  especially  perpen- 
dicular to  the  upper  and  under  surfaces  of  the  sheet. 
These  sometimes  assume  prismatic  forms,  dividing  the 
rock  into  columns,  as  is  so  frequently  to  be  observed  in 
basalt.  They  are  described  in  Book  IV.  Part  II.,  together 
with  other  forms  of  joints. 

Vapors  and  sublimations  of  a  lava-stream. 
— Besides  steam,  many  other  vapors,  absorbed  in  the  orig- 
inal subterranean  molten  magma,  escape  from  the  fissures 
of  a  lava-stream.  Such  vapors  are  copiously  disengaged  at 


83  See  Dana's  "Characteristics  of  Volcanoes,"  pp.  209,  332. 


DYNAMICAL    GEOLOGY  389 

fumaroles  (pp.  332,  334).  Among  the  exhalations,  chlorides 
abound,  particularly  chloride  of  sodium,  which  appears, 
not  only  in  fissures,  but  even  over  the  cooled  crust  of  the 
lava,  in  small  crystals,  in  tufts,  or  as  a  granular  and  even 
glassy  incrustation.  Chloride  of  iron  is  deposited  as  a 
yellow  coating  at  fumaroles,  where  also  bright  emerald- 
green  films  and  scales  of  chloride  of  copper  may  be  more 
rarely  observed.  Many  chemical  changes  take  place  in  the 
escape  of  these  vapors.  Thus  specular- iron,  either  the  re- 
sult of  the  mutual  decomposition  of  steam  and  iron- 
chloride,  or  of  the  oxidation  of  magnetite,  forms  abundant 
scales,  plates,  and  small  crystals  in  the  fumaroles  and  vesi- 
cles of  some  lavas.  Sal-ammoniac  also  appears  in  large 
quantity  on  many  lavas,  not  merely  in  the  fissures,  but 
also  on  the  upper  surface.  In  these  cases,  it  is  not  directly 
a  volcanic  product,  but  results  from  some  decomposition, 
possibly  from  the  gases  evolved  by  the  sudden  destruction 
of  vegetation.  It  has,  however,  been  observed  also  in  the 
crater  of  Etna,  where  the  co-operation  of  organic  substance 
is  hardly  conceivable,  and  where  perhaps  it  may  arise  from 
the  decomposition  of  aqueous  vapor,  whereby  a  combination 
is  formed  with  atmospheric  nitrogen.  Sulphur,  breislakite, 
szaboite,  tenorite,  alum,  sulphates  of  iron,  soda  and  potash, 
and  other  minerals  are  also  found. 

Slow  cooling  of  lava. — The  hardened  crust  of  a 
lava-stream  is  a  bad  conductor  of  heat.  Consequently,  the 
surface  of  the  stream  may  have  become  cool  enough  to  be 
walked  upon,  though  the  red-hot  mass  may  be  observed 
through  the  rents  to  lie  only  a  few  inches  below.  Many 
years,  therefore,  may  elapse  before  the  temperature  of  the 
whole  mass  has  fallen  to  that  of  the  surrounding  soil. 
Eleven  months  after  an  eruption  of  Etna,  Spallanzani 


390  TEXT-BOOK    OF    GEOLOGY 

could  see  that  the  lava  was  still  red-hot  at  the  bottom  of 
the  fissures,  and  a  stick  thrust  into  one  of  them  instantly 
took  fire.  The  Vesuvian  lava  of  1785  was  found  by  Breislak, 
seven  years  afterward,  to  be  still  hot  and  steaming  inter- 
nally, though  lichens  had  already  taken  root  on  its  surface. 
The  ropy  lava  erupted  by  Vesuvius  in  1858  was  observed 
by  the  author  in  1870  to  be  still  so  hot,  even  near  its  termi- 
nation, that  steam  issued  abundantly  from  its  rents,  many 
of  which  were  too  warm  to  allow  the  hand  to  be  held  in 
them,  and  three  years  later  it  was  still  steaming  abun- 
dantly. Hoffmann  records  that  from  the  lava  which  flowed 
from  Etna  in  1787,  steam  was  still  issuing  in  1830.  Yet 
more  remarkable  is  the  case  of  Jorullo,  in  Mexico,  which 
sent  out  lava  in  1759.  Twenty-one  years  later  a  cigar 
could  be  lighted  at  its  fissures;  after  44  years  it  was  still 
visibly  steaming;  and  even  in  1846,  that  is,  after  87  years 
of  cooling,  two  vapor-columns  were  still  rising  from  it.84 

This  extremely  slow  rate  of  cooling  has  justly  been  re- 
garded as  a  point  of  high  geological  significance,  in  regard 
to  the  secular  cooling  and  probable  internal  temperature  of 
our  globe.  Some  geologists  have  argued,  indeed,  that  if  so 
comparatively  small  a  portion  of  molten  matter  as  a  lava- 
stream  can  maintain  a  high  temperature  under  a  thin,  cold 
crust  for  so  many  years,  we  may,  from  analogy,  feel  little 
hesitation  in  believing  that  the  enormously  vaster  mass  of 
the  globe  may,  beneath  a  relatively  thin  crust,  still  continue 
in  a  molten  condition  within.  More  legitimate  deductions, 
however,  might  be  drawn  from  more  accurate  and  precise 
measurements  of  the  rate  of  loss  of  heat,  and  of  its  varia- 
tions in  different  lava-streams.  Lord  Kelvin,  for  instance, 

84  E.  Schleiden,  quoted  by  Naumann,  "Geognosie,"  i.  p.  160. 


DYNAMICAL    GEOLOGY  391 

has  suggested  that,  by  measuring  the  temperature  of  in- 
trusive masses  of  igneous  rock  in  coal- workings  and  else- 
where, and  comparing  it  with  that  of  other  non-volcanic 
rocks  in  the  same  regions,  we  might  obtain  data  for  calcu- 
lating the  time  which  has  elapsed  since  these  igneous  sheets 
were  erupted  (ante,  p.  94). 

Effects  of  lava-streams  on  superficial 
waters  and  topography. — In  its  descent,  a  stream 
of  lava  may  reach  a  water-course,  and,  by  throwing  itself 
as  an  embankment  across  the  stream,  may  pond  back  the 
water  and  form  a  lake.  Such  is  the  origin  of  the  pictur- 
esque Lake  Aidat  in  Auvergne.  Or  the  molten  current 
may  usurp  the  channel  of  the  stream,  and  completely  bury 
the  whole  valley,  as  has  happened  again  and  again  among 
the  vast  lava-fields  of  Iceland.  Few  changes  in  physiog- 
raphy are  so  rapid  and  so  enduring  as  this.  The  channel 
which  has  required,  doubtless,  many  thousands  of  years 
for  the  water  laboriously  to  excavate,  is  sealed  up  in  a  few 
hours  under  100  feet  or  more  of  stone,  and  another  vastly 
protracted  interval  may  elapse  before  this  newer  pile  is 
similarly  eroded.86 

By  suddenly  overflowing  a  brook  or  pool  of  water, 
molten  lava  sometimes  has  its  outer  crust  shattered  to 
fragments  by  a  sharp  explosion  of  the  generated  steam, 
while  the  fluid  mass  within  rushes  out  on  all  sides.8*  The 
lava  emitted  by  Mauna  Loa,  Hawaii,  in  the  spring  of  18§8 
flowed  out  to  sea,  and  added  half  a  mile  to  the  extent  of 


85  For  an  example  of  the  conversion  of  a  lava-buried  river-bed  into  a  hill-top 
by  long-continued  denudation,  see  Quart.  Journ.  Geol.  Soc.  1871,  p.  303. 

86  Explosions  of  this  nature  have  been  observed  on  Etna,  where  the  lava  has 
suddenly  come  in  contact  with  water  or  snow,  considerable  loss  of  life  being 
sometimes  the  result.     Sartorius  von  Waltershauaen  and  A.  von  Lasaulx,  "Der 
Aetna,"  i.  pp.  295,  300. 


392  TEXT-BOOK   OF   GEOLOGY 

the  island  at  that  point.  At  the  end  of  the  stream  three 
cinder-cones  formed  from  the  contact  of  the  lava  with  the 
water,  and  Captain  Button  calls  special  attention  to  the  fact 
that  not  only  in  this  instance,  but  in  other  examples  among 
the  Hawaiian  lavas  which  have  reached  the  sea,  there  is 
clear  evidence  of  the  formation  of  ordinary  volcanic  craters 
by  the  accidental  contact  of  lava  with  water.87  The  lavas 
of  Etna  and  Vesuvius  have  also  protruded  into  the  sea,  but, 
owing  probably  to  their  more  viscous  and  lithoid  condition 
and  lower  temperature,  they  do  not  seem  to  have  given  rise 
to  explosive  action  at  their  seaward  ends.  Thus  a  current 
from  the  latter  mountain  entered  the  Mediterranean  at  Torre 
del  Grreco  in  1794,  and  pushed  its  way  for  360  feet  outward, 
with  a  breadth  of  1100  and  a  height  of  15  feet.  So  quietly 
did  it  advance,  that  Breislak  could  sail  round  it  in  a  boat 
and  observe  its  progress. 

By  the  outpouring  of  lava,  two  important  kinds  of  geo- 
logical change  are  produced.  (1)  Stream-courses,  lakes, 
ravines,  valleys,  in  short,  all  the  minor  features  of  a  land- 
scape, may  be  completely  overwhelmed  under  a  thick  sheet 
of  lava.  The  drainage  of  the  district  being  thus  effectually 
altered,  the  numerous  changes  which  flow  from  the  opera- 
tions of  running  water  over  the  land  are  arrested  and  made 
to  begin  again  in  new  channels.  (2)  Considerable  altera- 
tions may  likewise  be  caused  by  the  effects  of  the  heat  and 
vapors  of  the  lava  upon  the  subjacent  or  contiguous  ground. 
Instances  have  been  observed  in  which  the  lava  has  actually 
melted  down  opposing  rocks,  or  masses  of  slags  on  its 
own  surface.  Interesting  observations,  already  referred  to 
(p.  386),  have  been  made  at  Torre  del  Greco  under  the  lava- 

81  U.  S.  Geol.  Report  for  1882-83,  p.  181. 


DYNAMICAL    GEOLOGY  393 

stream  which  overflowed  part  of  that  town  in  1794.  It  was 
found  that  the  window-panes  of  the  houses  had  been  devitri- 
fied  into  a  white,  translucent,  stony  substance;  that  pieces 
of  limestone  had  acquired  an  open,  sandy,  granular  texture, 
without  loss  of  carbon-dioxide,  and  that  iron,  brass,  lead, 
copper,  and  silver  objects  bad  been  greatly  altered,  some  of 
the  metals  being  actually  sublimed.  We  can  understand, 
therefore,  that,  retaining  its  heat  for  so  long  a  time,  a  mass 
of  lava  may  induce  many  crystalline  structures,  rearrange- 
ments, or  decompositions  in  the  rocks  over  which  it  comes 
to  rest,  and  proceeds  slowly  to  cool.  This  is  a  question  of 
considerable  importance  in  relation  to  the  behavior  of  an- 
cient lavas  which,  after  having  been  intruded  among  rocks 
beneath  the  surface,  have  subsequently  been  exposed  by 
denudation  (Book  IV.  Part  VII.). 

But,  on  the  other  hand,  the  exceedingly  trifling  change 
produced,  even  by  a  massive  sheet  of  lava,  has  often  been 
remarked  with  astonishment.  On  the  flank  of  Vesuvius, 
vines  and  trees  may  be  seen  still  flourishing  on  little  islets 
of  the  older  land-surface,  completely  surrounded  by  a  flood 
of  lava.  Dana  has  given  an  instructive  account  of  the  de- 
scent of  a  lava-stream  from  Kilauea  in  June,  1840.  Islet- 
like  spaces  of  forest  were  left  in  the  midst  of  the  lava,  many 
of  the  trees  being  still  alive.  Where  the  lava  flowed  round 
the  trees,  the  stumps  were  usually  consumed,  and  cylindrical 
holes  or  casts  remained  in  the  lava,  either  empty  or  filled 
with  charcoal.  In  many  cases,  the  fallen  crown  of  the  tree 
lay  near,  and  so  little  damaged  that  the  epiphytic  plants  on 
it  began  to  grow  again.  Yet  so  fluid  was  the  lava  that  it 
hung  in  pendent  stalactites  from  the  branches,  which  never- 
theless, though  clasped  round  by  the  molten  rock,  had  barely 
their  bark  scorched.  Again,  for  nearly  100  years  there  has 


394  TEXT-BOOK    OF   GEOLOGY 

lain  on  the  flank  of  Etna  a  large  sheet  of  ice,  which,  origi- 
nally in  the  form  of  a  thick  mass  of  snow,  was  overflowed  by 
lava,  and  has  thereby  been  protected  from  the  evaporation 
and  thaw  which  would  certainly  have  dissipated  it  long  ago, 
had  it  been  exposed  to  the  air.  The  heat  of  the  lava  has 
not  sufficed  to  melt  it.  Extensive  tracts  of  snow  were  like- 
wise overspread  by  lava  from  the  same  mountain  in  1879. 
In  other  cases,  snow  and  ice  have  been  melted  in  large  quan- 
tities by  overflowing  lava.  The  great  floods  of  water  which 
rushed  down  the  flank  of  Etna,  after  an  eruption  of  the 
mountain  in  the  spring  of  1755,  and  similar  deluges  at  Coto- 
paxi,  are  thus  explained. 

One  further  aspect  of  a  lava-stream  may  be  noticed  here 
— the  effect  of  time  upon  its  surface.  While  all  kinds  of 
lava  must,  in  the  end,  crumble  down  under  the  influence 
of  atmospheric  waste  and,  where  other  conditions  permit, 
become  coated  with  soil,  and  support  some  kind  of  vegeta- 
tion, yet  extraordinary  differences  may  be  observed  in  the 
facility  with  which  different  lava-streams  yield  to  this 
change,  even  on  the  flank  of  the  same  mountain.  Every 
one  who  ascends  the  slopes  of  Vesuvius  remarks  this  fact. 
After  a  little  practice,  it  is  not  difficult  there  to  trace  the 
limits  of  certain  lavas  even  from  a  distance,  in  some  cases 
by  their  verdure,  in  others  by  their  barrenness.  Five  hun- 
dred years  have  not  sufficed  to  clothe  with  green  the  still 
naked  surface  of  the  Catanian  lava  of  1381;  while  some  of 
the  lavas  of  the  present  century  have  long  given  footing  to 
bushes  of  furze."  Some  of  the  younger  lavas  of  Auvergne, 
which  certainly  flowed  in  times  anterior  to  those  of  history, 
are  still  singularly  bare  and  rugged.  Yet,  on  the  whole, 

88  On  the  weathering  of  the  Etna  lavas,  see  "Der  Aetna,"  ii.  p.  397. 


DYNAMICAL    GEOLOGY  395 

where  lava  is  directly  exposed  to  the  atmosphere,  without 
receiving  protection  from  occasional  showers  of  volcanic  ash, 
or  where  liable  to  be  washed  bare  by  heavy  torrents  of  rain, 
its  surface  decays  in  a  few  years  sufficiently  to  afford  soil  for 
stray  plants  in  the  crevices.  When  these  have  taken  root 
they  help  to  increase  the  disintegration;  at  last,  as  the  rock 
is  overspread,  the  traces  of  its  volcanic  origin  fade  away 
from  its  surface.  Some  of  the  Vesuvian  lavas  of  the  pres- 
ent century  already  support  vineyards. 

Elevation  and  Subsidence* — Proofs  of  elevation  are  fre- 
quent among  volcanic  vents  which,  lying  near  the  sea  and 
containing  marine  sediments  among  their  older  erupted  ma- 
terials, supply,  in  the  inclosed  marine  organisms,  evidence 
of  the  movement.  In  this  way,  it  is  known  that  Etna,  Vesu- 
vius, and  other  Mediterranean  volcanoes,  began  their  his- 
tory as  submarine  vents,  and  that  they  owe  their  present 
dimensions  not  only  to  the  accumulation  of  ejected  mate- 
rials, but  also,  to  some  extent,  to  an  elevation  of  the  sea- 
bottom. 

Proof  of  subsidence  is  less  easily  traced,  but  indications 
have  been  observed  of  a  sinking  of  the  ground  beneath  a 
volcanic  vent.  During  the  eruption  of  Santorin  in  1866-67, 
very  decided  but  extremely  local  subsidence  took  place 
near  the  vent  in  the  centre  of  the  old  crater.  The  discharge 
of  such  prodigious  quantities  of  material  may  tend  to  pro- 
duce cavernous  spaces  in  the  terrestrial  crust,  and  the  weight 
of  the  ejected  lavas  and  tuffs  may  still  further  contribute  to 
a  general  settlement  of  the  ground  around  a  volcanic  focus. 

If  we  consider  the  records  of  volcanic  action  in  past  geo- 
logical time  we  meet  with  many  proofs  that  it  took  place  in 
areas  where  the  predominant  terrestrial  movement  was  one 
of  subsidence.  Thus  among  the  Palaeozoic  systems  of  Brit- 


896  TEXT-BOOK    OF    GEOLOGY 

ain,  the  Cambrian,  Silurian,  Devonian,  Carboniferous,  and 
Permian  volcanoes  successively  appeared,  and  their  lavas 
and  tuffs  were  carried  down  and  buried  under  thousands  of 
feet  of  sedimentary  deposits.89 

Torrents  of  "Water  and  Mud. — We  have  seen  that  large 
quantities  of  water  accompany  many  volcanic  eruptions.  In 
some  cases,  where  ancient  crater-lakes  or  internal  reservoirs, 
shaken  by  repeated  detonations,  have  been  finally  disrupted, 
the  mud  which  has  thereby  been  liberated  has  issued  from 
the  mountain.  Such  "mud-lava"  {lava  d'acqua),  on  account 
of  its  liquidity  and  swiftness  of  motion,  is  more  dreaded  for 
destructiveness  than  even  the  true  melted  lavas.  On  the 
other  hand,  rain  or  melted  snow  or  ice,  rushing  down  the 
cone  and  taking  up  loose  volcanic  dust,  is  converted  into  a 
kind  of  mud  that  grows  more  and  more  pasty  as  it  descends. 
The  mere  sudden  rush  of  such  large  bodies  of  water  down 
the  steep  declivity  of  a  volcanic  cone  cannot  fail  to  effect 
much  geological  change.  Deep  trenches  are  cut  out  of  the 
loose  volcanic  slopes,  and  sometimes  large  areas  of  woodland 
are  swept  away,  the  de*bris  being  strewn  over  the  plains 
below. 

One  of  these  mud-lavas  invaded  Herculaneum  during 
the  great  eruption  of  79,  and  by  quickly  enveloping  the 
houses  and  their  contents,  has  preserved  for  us  so  many 
precious  and  perishable  monuments  of  antiquity.  In  the 
same  district,  during  the  eruption  of  1622,  a  torrent  of  this 
kind  poured  down  upon  the  villages  of  Ottajano  and  Massa, 
overthrowing  walls,  filling  up  streets,  and  even  burying 
houses  with  their  inhabitants.  During  the  great  eruption 
of  Cotopaxi,  in  June,  1877,  enormous  torrents  of  water  and 


Presidential  Addresses,  Quart.  Jouru.  Geol.  Soc.  xlvii.  (1891),  xlviii.  (1892). 


DYNAMICAL    GEOLOGY  397 

mud,  produced  by  the  melting  of  the  snow  and  ice  of  the 
cone,  rushed  down  from  the  mountain.  Huge  portions  of 
the  glaciers  of  the  mountain  were  detached  by  the  heat  of 
the  rocks  below  them  and  rushed  down  bodily,  breaking  up 
into  blocks.  The  villages  all  round  the  mountain  to  a  dis- 
tance of  sometimes  more  than  ten  geographical  miles  were 
left  deeply  buried  under  a  deposit  of  mud  mixed  with  blocks 
of  lava,  ashes,  pieces  of  wood,  lumps  of  ice,  etc."8  Many  of 
the  volcanoes  of  Central  and  South  America  discharge  large 
quantities  of  mud  directly  from  their  craters.  Thus,  in  the" 
year  1691,  Imbaburu,  one  of  the  Andes  of  Quito,  emitted 
floods  of  mud  so  largely  charged  with  dead  fish  that  pesti- 
lential fevers  arose  from  the  subsequent  effluvia.  Seven 
years  later  (1698),  during  an  explosion  of  another  of  the 
same  range  of  lofty  mountains,  Carguairazo  (14,706  feet), 
the  summit  of  the  cone  is  said  to  have  fallen  in,  while  tor- 
rents of  mud  containing  immense  numbers  of  the  fish  Pyme- 
lodus  Oyclopum,  poured  forth  and  covered  the  ground  over 
a  space  of  four  square  leagues.  The  carbonaceous  mud 
(locally  called  moya)  emitted  by  the  Quito  volcanoes  some- 
times escapes  from  lateral  fissures,  sometimes  from  the  cra- 
ters. Its  organic  contents,  and  notably  its  siluroid  fish, 
which  are  the  same  as  those  found  living  in  the  streams 
above  ground,  prove  that  the  water  is  derived  from  the  sur- 
face, and  accumulates  in  craters  or  underground  cavities 
until  discharged  by  volcanic  action.  Similar  but  even  moce 
stupendous  and  destructive  outpourings  have  taken  place 
from  the  volcanoes  of  Java,  where  wide  tracts  of  luxuriant 
vegetation  have  at  different  times  been  buried  under  masses 
of  dark  gray  mud,  sometimes  100  feet  thick,  with  a  rough 

»  Wolf,  Neues  Jahrb.  1878,  p.  133. 


398  TEXT-BOOK    OF   GEOLOGY 

hiliocky  surface  from  which  the  top  of  a  submerged  palm- 
tree  would  here  and  there  protrude. 

Between  the  destructive  effects  of  mere  water-torrents 
and  that  of  these  mud-floods  there  is,  of  course,  the  notable 
difference  that,  whereas  in  the  former  case  a  portion  of  the 
surface  is  swept  away,  in  the  latter,  while  sometimes  consid- 
erable demolition  of  the  surface  takes  place  at  first,  the  main 
result  is  the  burying  of  the  ground  under  a  new  tumultuous 
deposit  by  which  the  topography  is  greatly  changed,  not 
only  as  regards  its  temporary  aspect,  but  in  its  more  perma- 
nent features,  such  as  the  position  and  form  of  its  water- 
courses. 

Effects  of  the  dosing  of  a  Volcanic  Chimney — Sills  and 
Dikes* — A  study  of  the  volcanic  phenomena  of  former  geo- 
logical periods,  where  the  structure  of  the  interior  of  vol- 
canoes and  their  funnels  has  been  laid  bare  by  denudation, 
shows  that  in  many  cases  a  vent  becomes  plugged  up  by  the 
ascent  and  consolidation  of  solid  material  in  it,  while  yet 
the  eruptive  energy  of  the  volcano,  though  lessened,  has 
not  ceased.  A  time  is  reached  when  the  ascending  magma, 
impelled  by  pressure  from  below,  can  no  longer  overcome 
the  resistance  of  the  column  of  solid  lava  or  compacted  ag- 
glomerate which  has  sealed  up  the  orifice  of  discharge,  or  at 
least  when  it  can  more  easily  force  a  passage  for  itself  be- 
tween the  sedimentary  strata  on  which  the  whole  volcanic 
pile  may  rest,  or  between  the  lava  sheets  at  the  base  of  the 
pile,  or  into  fissures  in  either  or  both  of  these  groups. 
Hence  arise  intrusive  sheets  or  sills  and  dikes  or  veins  (see 
pp.  952,  958).  That  these  later  manifestations  of  volcanic 
energy  have  sometimes  taken  place  on  a  great  scale  is  shown 
by  the  number  and  size  of  the  sills  which  are  found  at  the 
base  of  the  Paleozoic  volcanic  groups  of  Britain.  This  fea- 


DYNAMICAL    GEOLOGY  399 

tare  is  a  remarkably  striking  feature  of  the  rocks  that  under- 
lie the  great  Lower  Silurian  volcanic  outflows  of  Arenig  and 
Cader  Idris  in  North  Wales.  It  recurs  so  frequently,  not 
only  among  Palaeozoic  volcanic  phenomena  but  quite  as 
markedly  among  those  of  Tertiary  age  in  the  British  Isles, 
that  it  must  be  regarded  as  marking  an  ordinary  phase  of 
volcanic  action.  Bat  it  remains  of  course  invisible  until  in 
the  progress  of  denudation  a  volcanic  cone  is  cut  down  to 
the  roots. 

Exhalations  of  Vapors  and  Gases. — A  volcano,  as  its  ac- 
tivity wanes,  may  pass  into  the  Solfatara  stage,  when  only 
volatile  emanations  are  discharged.  The  well-known  Sol- 
fatara near  Naples,  since  its  last  eruption  in  1198,  has  con- 
stantly discharged  steam  and  sulphurous  vapors.  The  island 
of  Volcano  has  now  passed  also  into  this  phase,  though  giv- 
ing vent  to  occasional  explosions.  Numerous  other  exam- 
ples occur  among  the  old  volcanic  tracts  of  Italy,  where  they 
have  been  termed  soffioni.  Steam,  escaping  in  conspicuous 
jets,  sulphuretted  hydrogen,  hydrochloric  acid,  and  carbonic 
acid  are  particularly  noticeable  at  these  orifices.  The  vapors 
in  rising  condense.  The  sulphuretted  hydrogen  partially 
oxidizes  into  sulphuric  acid,  which  powerfully  corrodes  the 
surrounding  rocks.  The  lava  or  tuff  through  which  the  hot 
vapors  rise  is  bleached  into  a  white  or  yellowish  crumbling 
clay,  in  which,  however,  the  less  easily  corroded  crystals 
may  still  be  recognized  in  situ.  At  the  same  time,  subli- 
mates of  sulphur  or  of  chlorides  may  be  formed,  or  the  sul- 
phuric acid  attacking  the  lime  of  the  silicates  gives  rise  to 
gypsum,  which  spreads  in  a  network  of  threads  and  veins 
through  the  hot,  steaming,  and  decomposed  mass.  In  this 
way,  at  the  island  of  Volcano,  obsidian  is  converted  into  a 
snow-white,  dull,  clay-stone-like  substance,  with  crystals  of 


400  TEXT-BOOK   OF   GEOLOGY 

sulphur  and  gypsum  in  its  crevices.  Silica  is  likewise  de- 
posited from  solution  at  many  orifices,  and  coats  the  altered 
rock  with  a  crust  of  chalcedony,  hyalite,  or  some  form  of 
siliceous  sinter.  As  the  result  of  this  action,  masses  of 
rock  are  decomposed  below  the  surface,  and  new  deposits 
of  alum,  sulphur,  sulphides  of  iron  and  copper,  etc.,  are 
formed  above  them.  Examples  have  been  described  from 
Iceland,  Lipari,  Hungary,  Terceira,  Teneriffe,  St.  Helena, 
and  many  other  localities."  The  lagoons  of  Tuscany  are 
basins  into  which  the  waters  from  suffioni  are  discharged, 
and  where  a  precipitation  of  their  dissolved  salts  takes  place. 
Among  the  substances  thus  deposited  are  gypsum,  sulphur, 
silica,  and  various  alkaline  salts;  but  the  most  important  is 
boracic  acid,  the  extraction  of  which  constitutes  a  thriving 
industry.  In  Chile  many  solfataras  occur  among  extinct 
volcanoes.'11 

Another  class  of  gaseous  emanations  betokens  a  condi- 
tion of  volcanic  activity  further  advanced  toward  final  ex- 
tinction. In  these,  the  gas  is  carbon-dioxide,  either  issuing 
directly  from  the  rock  or  bubbling  up  with  water  which  is 
often  quite  cold.  The  old  volcanic  districts  of  Europe  fur- 
nish many  examples.  Thus  on  the  shores  of  the  Laacher 
See — an  ancient  crater-lake  of  the  Eifel — the  gas  issues  from 
numerous  openings  called  moffette,  round  which  dead  insects, 
and  occasionally  mice  and  birds,  may  be  found.  In  the 
same  region  occur  hundreds  of  springs  more  or  less  charged 
with  this  gas.  The  famous  Valley  of  Deatli  in  Java  cou- 

91  Von  Buch,  "Ganar.  Inseln,"  p.  232.  Hoffmann.  Pogg.  Ann.  1832,  pp. 
38,  40,  60.  Bunsen,  Ann.  Chem.  Pharm.  1847  (Ixii.),  p.  10.  Darwin,  "Vol- 
canic Islands,"  p.  29.  The  name Propylite,  as  already  mentioned  (ante,  p.  293) 
lias  been  proposed  by  Rosenbusch  to  be  restricted  to  certain  andesitesand  allied 
rocks  altered  by  solfataric  action. 

94  Domeyko,  Ann.  Mines,  ix.  (7e.  aer.).  Large  numbers  of  solfataras  occur 
also  in  Iceland. 


DYNAMICAL    GEOLOGY  401 

tains  one  of  the  most  remarkable  gas-springs  in  the  world. 
It  is  a  deep,  boskj  hollow,  from  one  small  space  on  the  bot- 
tom of  which  carbon-dioxide  issues  so  copiously  as  to  form 
the  lower  stratum  of  the  atmosphere.  Tigers,  deer,  and 
wild-boar,  enticed  by  the  shelter  of  the  spot,  descend  and 
are  speedily  suffocated.  Many  skeletons,  including  those  of 
man  himself,  have  been  observed. 

As  a  distinct  class  of  gas-springs,  we  may  group  and  de- 
scribe here  the  emanations  of  volatile  hydrocarbons,  which, 
when  they  take  fire,  are  known  as  Fire- wells.  These  are 
not  of  volcanic  origin,  but  arise  from  changes  within  the 
solid  rocks  underneath.  They  occur  in  many  of  the  districts 
where  mud-volcanoes  appear,  as  in  northern  Italy,  on  the 
Caspian,  in  Mesopotamia,  in  southern  Kurdistan,  and  in 
many  parts  of  the  United  States.  It  has  been  observed 
that  they  frequently  rise  in  regions  where  beds  of  rock-salt 
lie  underneath,  and  as  that  rock  has  been  ascertained  often 
to  contain  compressed  gaseous  hydrocarbons,  the  solution  of 
the  rock  by  subterranean  water,  and  the  consequent  libera- 
tion of  the  gas,  has  been  offered  as  an  explanation  of  these 
fire-wells. 

In  the  oil  regions  of  Pennsylvania,  certain  sandy  strata 
occur  at  various  geological  horizons  whence  large  quantities 
of  petroleum  and  gas  are  obtained  (p.  254).  In  making  the 
borings  for  oil-wells,  reservoirs  of  gas  as  well  as  subterra- 
nean courses  or  springs  of  water  are  met  with.  When  the 
supply  of  oil  is  limited  but  that  of  gas  is  large,  a  contest 
for  possession  of  the  bore-hole  sometimes  takes  place  be- 
tween the  gas  and  water.  When  the  machinery  is  removed 
and  the  boring  is  abandoned,  the  contest  is  allowed  to  pro- 
ceed unimpeded  and  results  in  the  intermittent  discharge  of 
columns  of  water  and  gas  to  heights  of  130  feet  or  more. 


402  TEXT-BOOK    OF   GEOLOGY 

At  night,  when  the  gas  has  been  lighted,  the  spectacle  of 
one  of  these  "fire-geysers"  is  inconceivably  grand." 

Geysers. — Eruptive  fountains  of  hot  water  and  steam,  to 
which  the  general  name  of  Geysers  (i.e.  gushers)  is  given, 
from  the  examples  in  Iceland,  which  were  the  first  to  be 
seen  and  described,  mark  a  declining  phase  of  volcanic 
activity.  The  Great  and  Little  Geysers,  the  Strokkr  and 
other  minor  springs  of  hot  water  in  Iceland,  have  long  been 
celebrated  examples.  More  recently  another  series  has  been 
discovered  in  New  Zealand.  But  probably  the  most  re- 
markable and  numerous  assemblage  is  that  which  has  been 
brought  to  light  in  the  northwest  part  of  the  territory  of 
Wyoming,  and  which  has  been  included  within  the  "Yel- 
lowstone National  Park" — a  region  set  apart  by  the  Con- 
gress of  the  United  States  to  be  forever  exempt  from  settle- 
ment, and  to  be  retained  for  the  instruction  of  the  people. 
In  this  singular  region  the  ground  in  certain  tracts  is  honey- 
combed with  passages  which  communicate  with  the  surface 
by  hundreds  of  openings,  whence  boiling  water  and  steam 
are  emitted.  In  most  cases,  the  water  remains  clear,  tran- 
quil, and  of  a  deep  green-blue  tint,  though  many  of  the 
otherwise  quiet  pools  are  marked  by  patches  of  rapid  ebul- 
lition. These  pools  lie  on  mounds  or  sheets  of  sinter,  and 
are  usually  edged  round  with  a  raised  rim  of  the  same  sub- 
stance, often  beautifully  fretted  and  streaked  with  brilliant 
colors.  The  eruptive  openings  usually  appear  on  small, 

93  Ashburner,  Proc.  Amer.  Phil.  Soc.  xvii.  (1877),  p.  127.  Stowell's  Petro- 
leum Reporter,  15th  Sept.  1879.  Second  Geol.  Survey  of  Pennsylvania,  con- 
taining Reports  by  J.  Carll,  1877,  1880.  J.  S.  Newberry,  "The  First  Oil  Well." 
Harper's  Magazine,  Oct.  1890.  On  the  naphtha  districts  of  the  Caspian  Sea, 
Abich,  Jahrb.  Geol.  Reichs.  xxix.  (1879),  p.  165.  H.  Sjogren,  op.  cit.  xxxvii. 
(1887),  p.  47.  C.  Marvin,  "Region  of  Eternal  Fire,"  London,  1884.  See  also 
for  phenomena  in  Gallicia,  Jahrb.  Geol.  Reichs.  xv.  pp.  199,  351;  xvii.  p.  291; 
xviii.  p.  311;  xxxi.  (1881),  p.  131.  Proc.  Inst.  Civ.  Engineers,  xlii.  (1875), 
p.  343. 


DYNAMICAL    GEOLOGY 


403 


low,  conical  elevations  of  sinter,  from  each  of  which  one 
or  more  tubular  projections  rise.  It  is  from  these  irregular 
tube-like  excrescences  that  the  eruptions  take  place. 

The  term  geyser  is  restricted  to  active  openings  whence 
columns  of  hot  water  and  steam  are  from  time  to  time 
ejected;  the  non-eruptive  pools  are  only  hot  springs.  A 
true  geyser  should  thus  possess  an  underground  pipe  or 
passage,  terminating  at  the  surface  in  an  opening  built 


Fig.  52.— View  of  OKI  Faithful  Geyser,  and  others  in  the  distance,  Fire 
Hole  River,  Yellowstone  Park. 

round  with  deposits  of  sinter.  At  more  or  less  regular 
intervals,  rumblings  and  sharp  detonations  in  the  pipe  are 
followed  by  an  agitation  of  the  water  in  the  basin,  and  then 
by  the  violent  expulsion  of  a  column  of  water  and  steam  to 
a  considerable  height  in  the  air.  In  the  Upper  Fire  Hole 
basin  of  the  Yellowstone  Park,  one  of  the  geysers,  named 
"Old  Faithful"  (Fig.  52),  has  ever  since  the  discovery  of 
the  region  sent  out  a  column  of  mingled  water  and  steam 
every  sixty-three  minutes  or  thereabout.  The  column 
rushes  up  with  a  loud  roar  to  a  height  of  more  than  100 


404  TEXT-BOOK    OF    GEOLOGY 

feet,  the  whole  eruption  not  occupying  more  than  about 
live  or  six  minutes.  The  other  geysers  of  the  same  district 
are  more  capricious  in  their  movements,  and  some  of  them 
more  stupendous  in  the  volume  of  their  discharge.  The 
eruptions  of  the  Castle,  Giant,  and  Beehive  vents  are  mar- 
vellously impressive.94 

In  examining  the  Yellowstone  Geyser  region  in  1879, 
the  author  was  specially  struck  by  the  evident  indepen- 
dence of  the  vents.  This  was  shown  by-  their  very  differ- 
ent levels,  as  well  as  by  their  capricious  and  unsympathetic 
eruptions.  On  the  same  hill-slope,  dozens  of  quiet  pools, 
as  well  as  some  true  geysers,  were  noticed  at  different 
levels,  from  the  edge  of  the  Fire  Hole  River  up  to  a 
height  of  at  least  80  feet  above  it.  Yet  the  lower  pools, 
from  which,  of  course,  had  there  been  underground  con- 
nection between  the  different  vents,  the  drainage  should 
have  principally  discharged  itself,  were  often  found  to  be 
quiet  steaming  pools  without  outlet,  while  those  at  higher 
points  were  occasionally  in  active  eruption.  It  seemed  also 
to  make  no  difference  in  the  height  or  tranquillity  of  one 
of  the  quietly  boiling  caldrons,  when  an  active  projection 
of  steam  and  water  was  going  on  from  a  neighboring  vent 
on  the  same  gentle  slope. 

Bunsen  and  Deseloiseaux  spent  some  days  experimenting 
at  the  Icelandic  geysers,  and  ascertained  that  in  the  Great 
Geyser,  while  the  surface  temperature  is  about  212°  Fahr., 
that  of  lower  portions  of  the  tube  is  much  higher — a  ther- 
mometer giving  as  high  a  reading  as  266°  Fahr.  The  water 


94  See  Hayden's  Reports  for  1870  arid  for  1878,  in  the  latter  of  which  will  be 
found  a  voluminous  monograph  on  the  Hot  Springs  by  A.  C.  Peale ;  Comstock's 
Report  in  Jones's  Reconnoissance  of  N.  W.  Wyoming,  etc.,  1874.  The  de- 
posits of  hot  springs  are  further  referred  to  on  pp.  267,  809. 


DYNAMICAL    GEOLOGY  405 

at  a  little  depth  must  consequently  be  54°  above  the  normal 
boiling-point,  but  it  is  kept  in  the  fluid  state  by  the  pressure 
of  the  overlying  column.  At  the  basin,  however,  the  water 
cools  quickly.  After  an  explosion  it  accumulates  there,  and 
eventually  begins  to  boil.  The  pressure  on  the  column 
below  being  thus  relieved,  a  portion  of  the  superheated 
water  flashes  into  steam,  and  as  the  change  passes  down 
the  pipe,  the  whole  column  of  water  and  steam  rushes  out 
with  great  violence.  The  water  thereafter  gradually  col- 
lects again  in  the  pipe,  and  after  an  interval  of  some  hours 
the  operation  is  renewed.  The  experiments  made  by  Bun- 
sen  proved  the  source  of  the  eruptive  action  to  lie  in  the 
hot  part  of  the  pipe.  He  hung  stones  by  strings  to  different 
depths  in  the  funnel  of  the  geyser,  and  found  that  only 
those  in  the  higher  part  were  cast  out  by  the  rush  of  water, 
sometimes  to  a  height  of  100  feet,  while,  at  the  same  time, 
the  water  at  the  bottom  was  hardly  disturbed  at  all.  These 
observations  give  much  interest  and  importance  to  the  phe- 
nomena of  geysers  in  relation  to  volcanic  action.  They 
show  that  the  eruptive  force  in  geysers  is  steam;  that  the 
water  column,  even  at  a  comparatively  small  depth,  may 
have  a  temperature  considerably  above  212° ;  that  this  high 
temperature  is  local;  and  that  the  eruptions  of  steam  and 
water  take  place  periodically,  and  with  such  vigor  as  to 
eject  large  stones  to  a  height  of  100  feet.95 

The  hot  water  comes  up  with  a  considerable  percentage 
of  mineral  matter  in  solution.  According  to  the  analysis 
of  Sandberger,  water  from  the  Great  Geyser  of  Iceland 


9S  Comptes  Rendus,  xxiii.  (1846),  p.  934;  Pogg.  Annal.  Ixxii.  (1847),  p.  159; 
Ixxxiii.  (1851),  p.  197.  Ann.  Ghimie,  xxxviii.  (1853),  pp.  215,  385.  The  expla- 
nation proposed  for  the  phenomena  observed  at  the  Great  Geyser  is  probably  not 
applicable  in  those  cases  where  the  mere  local  accumulation  of  steam  in  suitable 
reservoirs  may  be  sufficient. 


406  TEXT-BOOK    OF   GEOLOGY 

contains  in  10,000  parts  the  following  proportions  of  ingre- 
dients: silica,  5-097;  sodium-carbonate,  1-939;  ammonium- 
carbonate,  0-083;  sodium-sulphate,  1-07;  potassium-sulphate, 
0-475;  magnesium-sulphate,  0-042;  sodium-chloride,  2-521; 
sodium-sulphide,  0-088;  carbonic  acid,  0-557  =  ll-872.96 

When  the  water  has  reached  the  surface,  it  deposits  the 
silica  as  a  sinter  on  the  surfaces  over  which  it  flows  or  on 
which  it  rests.97  The  deposit,  which  is  not  due  to  mere 
cooling  and  evaporation,  is  curiously  aided  by  the  presence 
of  living  algae  (postea,  p.  809).  It  naturally  takes  place 
fastest  along  the  margins  of  the  pools.  Hence  the  curiously 
fretted  rims  by  which  these  sheets  of  water  are  surrounded, 
and  the  tubular  or  cylindrical  protuberances  which  rise 
from  the  growing  domes.  Where  numerous  hot  springs 
have  issued  along  a  slope,  a  succession  of  basins  gives  a 
curiously  picturesque  terraced  aspect  to  the  ground,  as  at 
the  Mammoth  Springs  of  the  Yellowstone  Park  and  at  the 
now  destroyed  terraces  of  Rotamahana  in  New  Zealand. 

In  course  of  time,  the  network  of  underground  passages 
undergoes  alteration.  Orifices  that  were  once  active  cease 
to  erupt,  and  even  the  water  fails  to  overflow  them.  Sinter 
is  no  longer  formed  round  them,  and  their  surfaces,  exposed 
to  the  weather,  crack  into  fine  shaly  rubbish  like  com- 
minuted oyster-shells.  Or  the  cylinder  of  sinter  grows 
upward  until,  by  the  continued  deposit  of  sinter  and  the 
failing  force  of  the  geyser,  the  tube  is  finally  filled  up,  and 
then  a  dry  and  crumbling  white  pillar  is  left  to  mark  the 
site  of  the  extinct  geyser. 

"  Annal.  Chem.  und  Pharm.  1847,  p.  49.  A  series  of  detailed  analyses  of 
the  hot  springs  of  the  Yellowstone  National  Park  will  be  found  in  No.  47  of  the 
Bull.  U.  S.  Geol.  Surv.  1888. 

91  For  an  account  of  the  geyserite  of  the  Yellowstone  district,  see  papers  by 
W.  H.  Weed,  Anier.  Journ.  Sci.  xxxvii.  (1889),  and  9lh  Ann.  Rep.  U.  S.  GeoL 
Surv.  1890. 


DYNAMICAL    GEOLOGY  407 

Mud- Volcanoes*  —  These  are  of  two  kinds:  1st,  where 
the  chief  source  of  movement  is  the  escape  of  gaseous 
discharges;  2d,  where  the  active  agent  is  steam. 

(1)  Although  not  volcanic  in  the  proper  sense  of  the 
term,  certain  remarkable  orifices  of  eruption  may  be  no- 
ticed here,  to  which  the  names  of  mud-volcanoes,  salses,  air- 
volcanoes,  and  maccahtbas  have  been  applied  (Sicily,  the 
Apennines,  Caucasus,  Kertch,  Tamar).  These  are  conical 
hills  formed  by  the  accumulation  of  fine  and  usually  saline 
mud,  which,  with  various  gases,  is  continuously  or  intermit- 
tently given  out  from  the  orifice  or  crater  in  the  centre. 
They  occur  in  groups,  each  hillock  being  sometimes  less 
than  a  yard  in  height,  but  ranging  up  to  elevations  of  100 
feet  or  more.  Like  true  volcanoes,  they  have  their  periods 
of  repose,  when  either  no  discharge  takes  place  at  all,  or 
mud  oozes  out  tranquilly  from  the  crater,  and  their  epochs 
of  activity,  when  large  volumes  of  gas,  and  sometimes  col- 
umns of  flame,  rush  out  with  considerable  violence  and  ex- 
plosion, and  throw  up  mud  and  stones  to  a  height  of  several 
hundred  feet.  The  gases  play  much  the  same  part,  there- 
fore, in  these  phenomena  that  steam  does  in  those  of  true 
volcanoes.  They  consist  of  marsh*gas  and  other  hydrocar- 
bons, carbon-dioxide,  sulphuretted  hydrogen,  and  nitrogen, 
with  petroleum  vapors.  The  mud  is  usually  cold.  In  the 
water  occur  various  saline  ingredients,  among  which  com- 
mon salt  generally  appears;  hence  the  name,  Salses.  Naph- 
tha is  likewise  frequently  present.  Large  pieces  of  stone, 
differing  from  those  in  the  neighborhood,  have  been  ob- 
served among  the  ejections,  indicative  doubtless  of  a  some- 
what deeper  source  than  in  ordinary  cases.  Heavy  rains 
may  wash  down  the  minor  mud-cones  and  spread  out  the 
material  over  the  ground;  but  gas- bubbles  again  appear 


408  TEXT-BOOK    OF   GEOLOGY 

through  the  sheet  of  mud,  and  by  degrees  a  new  series 
of  mounds  is  once  more  thrown  up. 

There  can  be  little  doubt  that  this  type  of  mud- volcano 
is  to  be  traced  to  chemical  changes  in  progress  underneath. 
Dr.  Daubeny  explained  them  in  Sicily  by  the  slow  combus- 
tion of  beds  of  sulphur.  The  frequent  occurrence  of  naph- 
tha and  of  inflammable  gas  points,  in  other  cases,  to  the 
disengagement  of  hydrocarbons  from  subterranean  strata." 

(2)  The  second  class  of  mud-volcano  presents  itself  in 
true  volcanic  regions,  and  is  due  to  the  escape  of  hot  water 
and  steam  through  beds  of  tuff  or  some  other  friable  kind 
of  rock.  The  mud  is  kept  in  ebullition  by  the  rise  of  steam 
through  it.  As  it  becomes  more  pasty  and  the  steam  meets 
with  greater  resistance,  large  bubbles  are  formed  which 
burst,  and  the  more  liquid  mud  from  below  oozes  out  from 
the  vent.  In  this  way,  small  cones  are  built  up,  many  of 
which  have  perfect  craters  atop.  In  the  Geyser  tracts  of  the 
Yellowstone  region,  there  are  instructive  examples  of  such 
active  and  extinct  mud-vents.  Some  of  the  extinct  cones 
there  are  not  more  than  a  foot  high,  and  might  be  carefully 
removed  as  museum  specimens. 

Mud-volcanoes  occur*in  Iceland,  Sicily  (Maccaluba),  in 
many  districts  of  northern  Italy,  at  Tamar  and  Kertch,  at 
Baku  on  the  Caspian,  near  the  mouth  of  the  Indus,  and 
in  other  parts  of  the  globe." 


98  The  "burning  hills"  of  Turkestan  are  referred  to  the  subterranean  com- 
bustion of  beds  of  Jurassic  Coal.     J.  Muschketoff,  Neues  Jahrb.  1876,  p.  516. 

99  On  mud-volcanoes,  see  Bunsen,  Liebig's  Annual,  Ixiii.  (1847),  p.  1;  Abich, 
Mem.  Acad.  St.  Petersburg,  7e.  ser.  t.  vi.  No.  5,  ix.  No.  4;  Daubeny's  "Volca- 
noes," pp.  264,  539;  Buist,  Trans.  Bombay  Geograph.  Soc.  x.  p.  154;  Roberts, 
Journ.  Roy.  Asiatic  Soc.  1850;  De  Verneuil,  Mem.  Soc.  Geol.  France,  iii.  (1838), 
p.  4;  Stiffe,  Q.  J.  Geol.  Soc.  xxx.  p.  60;  Von  LasauLx,  Z.  Deutsch.  Geol.  Gea. 
xxxi.  p.  457 ;  Gumbel,  Sitzb.  Akad.  Munch.  1879;  F.  R.  Mallet,  Rec.  Geol.  Surv. 
India,  xi.  p.  188.     H.  Sjogren,  Jahrb.  Geol.  Reichsansc.  xxxvii.  (1887),  p.  233. 


DYNAMICAL    GEOLOGY  409 

§3.  Structure   of   Volcanoes 

We  have  now  to  consider  the  manner  in  which  the  vari- 
ous solid  materials  ejected  by  volcanic  action  are  built  up 
at  the  surface.  This  inquiry  will  be  restricted  here  to  the 
phenomena  of  modern  volcanoes,  including  the  active  and 
dormant,  or  recently  extinct,  phases.  Obviously,  however, 
in  a  modern  volcano  we  can  study  only  the  upper  and  ex- 
ternal portions,  the  deeper  and  fundamental  parts  being  still 
concealed  from  view.  But  the  interior  structure  has  been, 
in  many  cases,  laid  open  among  the  volcanic  products  of 
ancient  vents.  As  these  belong  to  the  architecture  of  the 
terrestrial  crust,  they  are  described  in  Book  IV.  The  stu- 
dent is  therefore  requested  to  take  the  descriptions  there 
given,  in  connection  with  the  foregoing  and  present  sec- 
tions, as  related  chapters  of  the  study  of  volcanism. 

Confining  attention  at  present  to  modern  volcanic  action, 
we  find  that  the  solid  materials  emitted  from  the  earth's 
interior  are  arranged  in  two  distinct  types  of  structure, 
according  as  the  eruptions  proceed  from  large  central  cones 
or  from  less  prominent  vents  connected  with  fissures.  In 
the  former  case,  volcanic  cones  are  produced;  in  the  lat- 
ter, volcanic  plateaus  or  plains.  The  type  of  the  volcanic 
cone,  or  ordinary  volcano,  is  now  the  most  abundant  and 
best  known. 

i.   Volcanic  Cones 

From  some  weaker  point  of  a  fissure,  or  from  a  vent 
opened  directly  by  explosion,  volcanic  discharges  of  gas 
and  vapors  with  their  liquid  and  solid  accompaniments 
make  their  way  to  the  surface  and  gradually  build  up 

a  volcanic  hill  or  mountain.     Occasionally,  eruptions  have 
OBOLOGY— Vol.  XXIX— 18 


410  TEXT-BOOK   OF   GEOLOGY 

proceeded  no  further  than  the  first  stage  of  gaseous  ex- 
plosion. A  caldron-like  cavity  has  been  torn  open  in  the 
ground,  and  ejected  fragments  of  the  solid  rocks,  through 
which  the  explosion  has  emerged,  have  fallen  back  into 
and  round  the  vent.  Subsequently,  after  possible  subsi- 
dence of  the  fragmentary  materials  in  the  vent,  and  even 
of  the  sides  of  the  orifice,  water  supplied  by  rain  and  filter- 
ing from  the  neighboring  ground  may  partially,  or  wholly, 
fill  up  the  cavity,  so  as  to  produce  a  lake  either  with 
or  without  a  superficial  outlet.  Under  favorable  circum- 
stances, vegetation  creeping  over  bare  earth  and  stone  may 
so  conceal  all  evidence  of  the  original  volcanic  action  as  to 
make  the  quiet  sheet  of  water  look  as  if  it  had  always  been 
an  essential  part  of  the  landscape.  Explosion-lakes  (Crater- 
lakes)  of  this  kind  occur  in  districts  of  extinct  volcanoes, 
as  in  the  Eifel  (maare),  central  Italy,  and  Auvergne.  The 
crateriform  hollow  called  the  Gour  de  Tazenat,  in  Velay, 
has  a  diameter  of  half  a  mile  and  lies  in  the  granite,  while 
another  cavity  near  Confolens,  on  the  left  bank  of  the 
Loire,  has  also  been  blown  out  of  the  granite  and  has 
given  passage  to  no  volcanic  materials,  but  only  to  broken- 
up  granite.100  Other  illustrations  in  central  France  are  to 
be  found  in  the  Lakes  of  Pavin,  Mont  Sineire,  Chauvet, 
Beurdouse,  Champedaze  and  La  Grodival.101  A  remarkable 
example  is  supplied  by  the  Lonar  Lake  in  the  Indian  penin- 
sula, half-way  between  Bombay  and  Nagpur.  It  lies  in  the 
midst  of  the  volcanic  plateau  of  the  Deccan  traps,  which 
extend  around  it  for  hundreds  of  miles  in  nearly  flat  beds 
that  slightly  dip  away  from  the  lake.  An  almost  circular 


100  Tournaire,  Bull.   Soc.   Geol.   France,   xxvi.   (1869),  p.   1166;   Daubree, 
Comptes  rend.   1890,  p.  859. 

101  Scrope,  "Volcanoes  of  Central  France,"  pp.  81,  143,  144. 


DYNAMICAL    GEOLOGY  411 

depression,  rather  more  than  a  mile  in  diameter,  and  from 
300  to  400  feet  deep,  contains  at  the  bottom  a  shallow  lake 
of  bitter  saline  water,  depositing  crystals  of  trona  (native 
carbonate  of  soda),  the  nitrum  of  the  ancients.  Except  to 
the  north  and  northeast,  it  is  encircled  with  a  raised  rim 
of  irregularly  piled  blocks  of  basalt,  identical  with  that  of 
the  beds  through  which  the  cavity  has  been  opened.  The 
rim  never  exceeds  100  feet,  and  is  often  not  more  than  40 
or  60  feet  in  height,  and  cannot  contain  a  thousandth  part 
of  the  material  which  once  filled  the  crater.  No  other  evi- 
dence of  volcanic  discharge  from  this  vent  is  to  be  seen. 
Some  of  the  contents  of  the  cavity  may  have  been  ejected 
in  fine  particles,  which  have  subsequently  been  removed  by 
denudation;  but  it  seems  more  probable  that  the  existence 
of  the  cavity  is  mainly  due  to  subsidence  after  the  original 
explosion.1" 

In  most  cases,  explosions  are  accompanied  by  the  ex- 
pulsion of  so  much  solid  material  that  a  cone  gathers  round 
the  point  of  emission.  As  the  cone  increases  in  height,  by 
successive  additions  of  ashes  or  lava  to  its  surface,  these 
volcanic  sheets  are  laid  down  upon  progressively  steeper 
slopes.  The  inclination  of  beds  of  lava,  which  must  have 
originally  issued  in  a  more  or  less  liquid  condition,  offered 
formerly  a  difficulty  to  observers,  and  suggested  the  fa- 
mous theory  of  Elevation-craters  (Erhebungskratere)  of  L. 
von  Buch,108  Elie  de  Beaumont,1*4  and  other  geologists. 


104  This  cavity  may  possibly  mark  one  of  the  vents  from  which  the  basalt 
Hoods  issued.  On  explosion-craters  and  lakes,  see  Scrope's  "Volcanoes." 
Lecoq,  "Epoquea  geologiques  de  1'Auvergne,"  tome  iv. ;  compare  also  Vogel- 
sang, "Vulcane  der  Eifel,"  and  in  Neues  Jahrb.  1870,  pp.  199,  326,  460.  On 
Lonar  Lake,  see  Malcolmson,  Trans.  Geol.  Soc.  2d  ser.  v.  p.  562.  Medlicott  and 
Stanford's  "Geology  of  India,"  p.  379. 

103  Pogg.  Ann.  ix.,  x.,  xivii.  p.  169. 

104  Bull.  Soc.  Geol.  France,  iv.  p.  357.     Ann.  dea  Mines,  ix.  and  x. 


412  TEXT-BOOK   OF   GEOLOGY 

According  to  this  theory,  the  conical  shape  of  a  volcanic 
cone  arises  mainly  from  an  upheaval  or  swelling  of  the 
ground,  round  the  vent  from  which  the  materials  are  finally 
expelled.  A  portion  of  the  earth's  crust  (represented  in 
Fig.  53  as  composed  of  stratified  deposits,  a  b  g  h]  was 
believed  to  have  been  pushed  up  like  a  huge  blister,  by 
forces  acting  from  below  (at  c)  until  the  summit  of  the  dome 
gave  way  and  volcanic  materials  were  emitted.  At  first 
these  might  only  partially  fill  the  cavity  (as  at/),  but  sub- 
sequent eruptions,  if  sufficiently  copious,  would  cover  over 


Fig.  58.— Section  illustrative  of  the  Elevation-crater  Theory. 

the  truncated  edges  of  the  pre-volcanic  rocks  (as  at  g  ^),  and 
would  be  liable  to  further  upheaval  by  a  renewal  of  the 
original  upward  swelling  of  the  site. 

It  was  a  matter  of  prime  importance  in  the  interpretation 
of  volcanic  action  to  have  this  question  settled.  To  Poulett 
Scrope,  Constant  Provost,  and  Lyell,  belong  the  merit  of 
disproving  the  Elevation-crater  theory.  Scrope  showed  con- 
clusively that  the  steep  slope  of  the  lava-beds  of  a  volcanic 
cone  was  original.1"  Constant  Provost  pointed  out  that 
there  was  no  more  reason  why  lava  should  not  consolidate 

106  "Considerations  oa  Volcanoes,"  1825.  Quart.  Journ.  Geol.  Soc.  xH. 
p.  326. 


DYNAMICAL    GEOLOGY 


413 


on  steep  slopes  than  that  tears  or  drops  of  wax  should  not 
do  so.10"  Lyell,  in  successive  editions  of  his  works,  and 
subsequently  by  an  examination  of  the  Canary  Islands 
with  Hartung,  brought  forward  cogent  arguments  against 
the  Elevation-crater  theory. lif  A  comparison  of  Fig.  53 
with  Fig.  54  will  show  at  a  glance  the  difference  between 


Pig.  54.— Diagram-sectioii  of  a  normal  Volcano. 

*  x,  Pre-volcanic  platform,  supposed  here  to  consist  of  upraised  stratified  rocks, 
broken  through  by  the  funnel  /,  from  which  the  cone  of  volcanic  materials  c  c 
has  been  erupted.  Inside  the  crater  u,  previously  cleared  by  some  great  explo- 
sion, a  minor  cone  may  be  formed  during  feebler  phases  of  volcanic  action,  and 
this  inner  cone  may  increase  in  size  until  the  original  cone  is  built  up  again,  as 
shown  by  the  dotted  lines. 

this  theory  and  the  views  of  volcanic  structure  now  uni- 
versally accepted.  The  steep  declivities  on  which  lava 
can  actually  consolidate  have  been  referred  to  on  p.  386. 
The  conical  form  of  a  volcano  is  that  naturally  assumed 
by  a  self-supporting  mass  of  coherent  material.  It  varies 
slightly  according  to  the  nature  of  the  materials  of  the  cone, 
the  progress  of  atmospheric  denudation,  the  position  of  the 
crater,  the  direction  in  which  materials  are  ejected,  the  force 
and  direction  of  the  wind  during  an  eruption,  the  growth 


m  Comptes  Rendus,  i.  (1835),  460;  xli.  (1855),  p.  919.     Geol.  Soc.  Prance: 
Memoires,  ii.   p.   105,  and  Bull.  xiv.  217.     Societe  Philom.  Paris,  Proc.  Verb. 


1843,  p.  13. 

IM  Phil.  Trans,  1858,  p.  703. 
4DO-422. 


See  the  remarks  of  Fouque,  "Santorin,"  pp. 


414  TEXT-BOOK    OF   GEOLOGY 

of  parasitic  cones,  and  the  collapse  due  to  the  dying  out  of 
volcanic  energy.188 

The  cone  grows  by  additions  made  to  its  surface  during 
successive  eruptions,  and  though  liable  to  great  local  varia- 
tion of  contour  and  topography,  preserves  its  general  form 
with  singular  persistence.  Many  exaggerated  pictures  have 
been  drawn  of  the  steepness  of  slope  in  volcanic  cones,  but 
it  is  obvious  that  the  angle  cannot  as  a  whole  exceed  the 
maximum  inclination  of  repose  of  the  detrital  matter  ejected 
from  the  central  chimney.108  A  series  of  profiles  of  volcanic 
cones  taken  from  photographs  shows  how  nearly  they  ap- 
proach to  a  common  average  type.110  One  of  the  most 
potent  and  constant  agencies  in  modifying  the  outer  forms 
of  these  cones  is  undoubtedly  to  be  found  in  rain  and  tor- 
rents, which  sweep  down  the  loose  detritus  and  excavate 
ravines  on  the  declivities  till  a  cone  may  be  so  deeply 
trenched  as  to  resemble  a  half -opened  umbrella.111 

The  crater  doubtless  owes  its  generally  circular  form  to 
the  equal  expansion  in  all  directions  of  the  explosive  vapors 
from  below.  In  some  of  the  mud-cones  already  noticed,  the 
crater  is  not  more  than  a  few  inches  in  diameter  and  depth. 
From  this  minimum,  every  gradation  of  size  may  be  met 
with,  up  to  huge  precipitous  depressions,  a  mile  or  more 
in  diameter,  and  several  thousand  feet  in  depth.  In  the 
crater  of  an  active  volcano,  emitting  lava  and  scoriae,  like 
Vesuvius,  the  walls  are  steep,  rugged  cliffs  of  scorched  and 

108  J.  Milne,  Geol.  Mag.  1878,  p.  339;  1879,  p.  506.     Seismolog.  Soc.  Japan, 
ix.  p.  179.     G.  P.  Becker,  Amer.  Journ.  Sci.  xxx.  1885,  p.  283. 

109  Cotopaxi  is  a  notable  example  of  such  exaggerated  representation.     Mr. 
"Whymper  found  that  the  general  angles  of  the  northern  and  southern  slopes  of 
the  cone  were  rather  less  than  30*  ("Travels  amongst  the  Great  Andes,"  p.  123). 
Humboldt  depicted  the  angle  as  one  of  60°  1 

110  See  Milne,  Seism.  Soc.  Japan,  ix.,  and  Geol.  Mag.  1878,  plate  ix. 

111  On  the  denudation  of  volcanic  cones,  see  H.  J.  Johnston-Lavis,  Q.  J.  Geol. 
Soc.  xl.  p.  103. 


DYNAMICAL    GEOLOGY  415 

blasted  rock — red,  yellow,  and  black.  Where  the  material 
erupted  is  only  loose  dust  and  lapilli,  the  sides  of  the  crater 
are  slopes,  somewhat  steeper  than  those  of  the  outside  of 
the  cone. 

The  crater-bottom  of  an  active  volcano  of  the  first  class, 
when  quiescent,  forms  a  rough  plain  dotted  over  with  hil- 
locks or  cones,  from  many  of  which  steam  and  hot  vapors 
are  ever  rising.  At  night,  the  glowing  lava  may  be  seen 
lying  in  these  vents,  or  in  fissures,  at  a  depth  of  only  a  few 
feet  from  the  surface.  Occasional  intermittent  eruptions 
take  place  and  miniature  cones  of  slag  and  scoriae  are 
thrown  up.  In  some  instances,  as  in  the  vast  crater  of 
Gurung  Tengger,  in  Java,  the  crater-bottom  stretches  out 
into  a  wide  level  waste  of  volcanic  sand,  driven  by  the  wind 
into  dunes  like  those  of  the  African  deserts. 

A  volcano  commonly  possesses  one  chief  crater,  oftem 
also  many  minor  ones,  of  varying  or  of  nearly  equal  size. 
The  volcano  of  the  Isle  of  Bourbon 
(or  Reunion)  has  three  craters.114  Not 
infrequently  craters  appear  successive- 
ly, owing  to  the  blocking  up  of  the 
pipe  below.  Thus  in  the  accompany- 
ing plan  of  the  volcanic  cone  of  the 
island  of  Volcanello  (Fig.  55),  one  of  Fig.  55.— Plan  of  voicaneiio, 

0          '     .  showing  three  successive 

the   Lipari    group,    the   volcanic    fun-  craters. 

nel  has  shifted  its  position  twice,  so  that  three  craters 
have  successively  appeared  upon  the  cone,  and  partially 
overlap  each  other.  It  may  be  from  this  cause  that  some 


114  For  receut  information  regarding  this  volcanic  island,  see  B.  von  Drasche, 
in  Verhandl.  Geol.  Reichsanst.  1875,  p.  266,  and  in  Tschermak'a  Min.  Mittheil. 
1875  (3),  p.  217  (4),  p.  39,  and  his  work  "Die  Insel  Reunion  (Bourbon),"  4to, 
Vienna,  1878.  C.  Velain.  "Description  geologique  de  la  Presqu'ile  d'Adeu,  de 
1'Ile  de  la  Reunion,  etc.,"  Paris,  4to,  1878;  and  his  work,  "Lea  Volcans, "  1884. 


416  TEXT-BOOK    OF   GEOLOGY 

volcanic  mountains  are  now  destitute  of  craters,  or  in  other 
cases,  because  the  lava  has  welled  up  in  dome  form  covered 
perhaps  with  masses  of  scoriae,  but  without  the  production 
of  a  definite  crater.  Mount  Ararat,  for  example,  is  said  to 
have  no  crater;  but  so  late  as  the  year  1840  a  fissure  opened 
on  its  side  whence  a  considerable  eruption  took  place.  The 
trachytic  puys  of  Auvergne  are  dome-shaped  hills  without 
craters. 

Though  the  interior  of  modern  volcanic  cones  can  be  at 
the  best  but  very  partially  examined,  the  study  of  the  sites 
of  long-extinct  cones,  laid  bare  after  denudation,  shows  that 
subsidence  of  the  ground  has  commonly  taken  place  at  and 
round  a  vent.  Evidence  of  subsidence  has  also  been  ob- 
served at  some  modern  volcanoes  (ante,  p.  395).  Theoreti- 
cally two  causes  may  be  assigned  for  this  structure.  In  the 
first  place,  the  mere  piling  up  of  a  huge  mass  of  material 
round  a  given  centre  tends  to  press  down  the  rock  under- 
neath, as  some  railway  embankments  may  be  observed  to 
have  done.  This  pressure  must  often  amount  to  several 
hundred  tons  on  the  square  foot.  In  the  second  place,  the 
expulsion  of  volcanic  material  to  the  surface  may  leave 
cavities  underneath,  into  which  the  overlying  crust  will 
naturally  gravitate.  These  two  causes  combined,  as  sug- 
gested by  Mr.  Mallet,  afford  a  probable  explanation  of  the 
saucer-shaped  depressions  in  which  many  ancient  and  some 
modern  vents  appear  to  lie.118 

The  following  are  the  more  important  types  of  volcanic 
cones:114 

113  Mallet,  Q.  J.  Geol.  Soc.  xxxiii.  p.  740.     See  also  the  account  of  "Volcanic 
Necks,"  in  Book  IV.  Part  VII. 

114  Von  Seebach  (Z.  Deutsch.  Geol.  Ges.  xviii.  644)  distinguished  two  vol- 
canic types.     1st,  Bedded  Volcanoes  (Strato-Vulkane),  composed  of  successive 
sheets   of   lava   and   tuffs,    and   embracing   the   great   majority  of   volcanoes. 
2d,  Dome  Volcanoes,  formiug  hills  composed  of  homogeneous  protrusions  of 


DYNAMICAL    GEOLOGY  417 

1.  Cones  of  Non-volcanic  Materials. — These  are  due  to  the 
discharge  of  steam  or  other  aeriform  product  through  the 
solid  crust  without  the  emission  of  any  true  ashes  or  lava. 
The  materials  ejected  from  the  cavity  are  wholly,  or  almost 
wholly,  parts  of  the  surrounding  rocks  through  which  the 
volcanic  pipe  has  been  drilled.  Some  of  the  cones  surround- 

_•  ^sJsSBKSte*-.- 


Fig.  56— View  of  the  Tuff-cones  of  Auvergne,  taken  from  the  top  of  the  cone  and 
crater  of  Puy  Pariou. 

ing  the  crater  lakes  (maare)  of  the  Eifel  consist  chiefly  of 
fragments  of  the  underlying  Devonian  slates  (pp.  341,  363). 
2.  Tuff-Cones,  Cinder-Cones. — Successive  eruptions  of  fine 
dust  and  stones,  often  rendered  pasty  by  mixture  with  the 
water  so  copiously  condensed  during  an  eruption,  form  a 
cone  in  which  the  materials  are  solidified  by  pressure  into 
tuff.  Cones  made  up  only  of  loose  cinders,  like  Monte 
Nuovo  in  the  Bay  of  Baiae,  often  arise  on  the  flanks  or 
round  the  roots  of  a  great  volcano,  as  happens  to  a  small 
extent  on  Vesuvius,  and  on  a  larger  scale  upon  Etna.  They 

lava,  with  little  or  no  accompanying  fragmentary  discharges,  without  cratera 
or  chimneys,  or  at  least  with  only  minor  examples  of  these  volcanic  features. 
He  believed  that  the  same  volcano  might  at  different  periods  in  its  history  be- 
long to  one  or  other  of  these  types — the  determining  cause  being  the  nature  of 
the  erupted  lava,  which,  in  the  case  of  the  dome  volcanoes,  is  less  fusible  and 
more  viscid  than  in  that  of  the  bedded  volcanoes.  (See  below,  under  "Lava- 
cones.") 


418 


TEXT-BOOK    OF   GEOLOGY 


likewise  occur  by  themselves  apart  from  any  lava-producing 
volcano,  though  usually  they  afford  indications  that  columns 
of  lava  have  risen  in  their  'funnels,  and  even  now  and  then 
that  this  lava  has  reached  the  surface. 

The  cones  of  the  Eifel  district  have  long  been  celebrated 
for  their  wonderful  perfection.  Though  small  in  size,  they 
exhibit  with  singular  clearness  many  of  the  leading  features 
of  volcanic  structure.  Those  of  Auvergne  are  likewise  ex- 
ceedingly instructive.116  The  high  plateaus  of  Utah  are 
dotted  with  hundreds  of  small  volcanic  cinder-cones,  the 
singular  positions  of  which,  close  to  the  edge  of  profound 
river-gorges  and  on  the  upthrow  side  of  faults,  have  already 
(p.  348)  been  noticed.  Among  the  Carboniferous  volcanic 
rocks  of  central  Scotland  the  stumps  of  ancient  tuff-cones, 
frequently  with  a  central  core  of  basalt,  or  with  dikes  and 
veins  of  that  rock,  are  of  common  occurrence.1" 

The  materials  of  a  tuff-cone  are  arranged  in  more  or  less 
regularly  stratified  beds.  On  the  outer  side,  they  dip  down 
the  slopes  of  the  cone  at  the  average  angle  of  repose,  which 


Fig.  57.— Section  of  the  crater-rim  of  the  Island  of  Volcano. 
a,  Older  tuff;  6  6,  younger  ashes;  the  crater  lies  to  the  right. 

may  range  between  30°  and  40°.  From  the  summit  of  the 
crater-lip  they  likewise  dip  inward  toward  the  crater-bottom 
at  similar  angles  of  inclination  (Pig.  57). 

3.  Mud-Cones  resemble  tuff -cones  in  form,  but  are  usually 
smaller  in  size  and  less  steep.  They  are  produced  by  the 
hardening  of  successive  outpourings  of  mud  from  the  ori- 
fices already  described  (p.  407).  In  the  region  of  the  Lower 
Indus,  where  they  are  abundantly  distributed  over  an  area 


116  For  Auvergne,  see  works  cited  on  p.  374.  For  the  Eifel,  consult  Hib- 
bert,  "History  of  the  Extinct  Volcanoes  of  the  Basin  of  Neuwied  on  the  Lower 
Ehine,"  Edin.  1832.  Von  Dechen,  "Geognostischer  Fuhrer  zu  dem  Laacher 
See,"  Bonn,  1864.  "Geognostischer  Fuhrer  in  das  Siebengebirge  am  Rhein," 
Bonn,  1861. 

"•  Trans.  Roy.  Soc.  Edin.  xxix.  p.  455.     See  postea,  Book  IV.  Part  VII. 


DYNAMICAL    GEOLOGY  419 

of  1000  square  miles,  some  of  them  attain  a  height 
of  400  feet,  with  craters  30  yards  across.1" 

4.  Lava-cones. — Yolcanic  cones  composed  en- 
tirely of  lava  are  comparatively  rare,  out  occur 
in  some  younger  Tertiary  and  modern  volcanoes. 
Fouque  describes  the  lava  of  1866  at  Santorin 
as  having  formed  a  dome-shaped  elevation,  flow- 
ing out  quietly  and  rapidly  without  explosions. 
After  several  days,  however,  its  emission  was  ac- 
companied with  copious  discharges  of  fragmentary 
materials  and  the  formation  of  several  crateriform 
mouths  on  the  top  of  the  dome.  Where  lava  pos- 
sesses extreme  liquidity,  and  gives  rise  to  little 
or  no  fragmentary  matter,  it  may  build  up  a  flat 
cone  as  in  the  remarkable  examples  described  by 


Fig.  59.— Plan  of  Lava-caldron,  Kilauea,  Hawaii  (Dana,  1841  u»). 

Dana  from  the  Hawaii  Islands.118  On  the  summit 
of  Mauua  Loa  (Fig.  58),  a  flat  lava-cone  13,760 
feet  above  the  sea,  lies  a  crater,  which  in  its  deep- 
est part  is  about  8,000  feet  broad,  with  vertical 
walls  of  stratified  lava  rising  on  one  side  to  a 
height  of  784  feet  above  the  black  lava- plain  of 
the  crater-bottom.  From  the  edges  of  this  ele- 
vated caldron,  the  mountain  slopes  outward  at  an 
angle  of  not  more  than  6°,  until,  at  a  level  of 
about  10,000  feet  lower,  its  surface  is  indented 

m  Lyell,  "Principles,"  ii.  p.  77. 

118  In  Wilkes's  Report  of  U.  S.  Exploring  Expedition,  1838-42,  and  Dana's 
"Characteristics  of  Volcanoes."     See  the  works  cited  on  p.  350. 

119  For  more  recent  maps  showing  the  variations  of  this  crater,  see  Dana's 
"Characteristics. ' ' 


420  TEXT-BOOK    OF   GEOLOGY 

by  the  vast  pit-crater,  Kilauea,  about  two  miles  long,  and 
nearly  a  mile  broad.  So  low  are  the  surrounding  slopes 
that  these  vast  craters  have  been  compared  to  open  quarries 
on  a  hill  or  moor.  The  bottom  of  Kilauea  is  a  lava-plain, 
dotted  with  lakes  of  extremely  fluid  lava  in  constant  ebul- 
lition. The  level  of  the  lava  has  varied,  for  the  walls  sur- 
rounding the  fiery  flood  consist  of  beds  of  similar  lava,  and 
are  marked  by  ledges  or  platforms  (Fig.  59)  indicative  of 
former  successive  heights  of  lava,  as  lake  terraces  show 
former  levels  of  water.  In  the  accompanying  section  (Fig. 
60)  the  walls  rising  above  the  lower  pit  (p  p')  were  found  to 
be  342  feet  high,  those  bounding  the  higher  terrace  (o  n  n'  o') 
were  660  feet  high,  all  being  composed  of  innumerable  beds 
of  lava,  as  in  cliffs  of  stratified  rocks.  Much  of  the  bottom 
of  the  lower  lava-plain  has  been  crusted  over  by  the  solidifi- 
cation of  the  molten  rock.  But  large  areas,  which  shift 


Fig.  60.— Section  of  Lava-terraces  in  Kilauea  (Dana). 

their  position  from  time  to  time,  remain  in  perpetual  rapid 
ebullition.  The  glowing  flood,  as  it  boils  up  with  a  fluidity 
more  like  that  of  water  than  what  is  commonly  shown  by 
molten  rock,  surges  against  the  surrounding  terrace  walls. 
Large  segments  of  the  cliffs,  undermined  by  the  fusion  of 
their  base,  fall  at  intervals  into  the  fiery  waves  and  are  soon 
melted.  Recent  observations  by  Captain  Button  point  to  a 
diminution  of  the  activity  of  this  lava-crater.  In  Iceland, 
and  in  the  Western  Territories  of  North  America,  low  domes 
of  lava  appear  to  mark  the  vents  from  which  extensive 
basalt  floods  have  issued. 

Where  the  lava  assumes  a  more  viscid  character,  as  in 
trachyte  and  liparite,  dome-shaped  eminences  may  be  pro- 
truded. As  the  mass  increases  in  size  by  the  advent  of 
fresh  material  injected  from  below,  the  outer  layer  will  be 
pushed  outward,  and  successive  shells  will  in  like  manner 
be  enlarged  as  the  eruption  advances.  On  the  cessation  of 
discharges,  we  may  conceive  that  a  volcanic  hill  formed  in 
this  way  will  present  an  onion-like  arrangement  of  its  com- 
ponent sheets  of  rock.  More  or  less  perfect  examples  of  this 
structure  have  been  observed  in  Bohemia,  Auvergne,  and 


DYNAMICAL    GEOLOGY 


421 


the  Eifel.180  The  trachytic  domes  of  Auvergne  form  a  con- 
spicuous feature  among  the  cinder  cones  of  that  region. 
Huge  conical  protuberances  of  granophyre  occur  among  the 
Tertiary  volcanic  rocks  of  the  Inner  Hebrides,  and  similar 
hills  of  liparite  rise  through  the  basalts  of  Iceland. 

5.  Cones  of  Tuff  and  Lava. — This  is  by  far  the  most  abun- 
dant type  of  volcanic  structure,  and  includes  the  great  vol- 
canoes of  the  globe.  Beginning,  perhaps,  as  mere  tuff-cones, 
these  eminences  have  gradually  been  built  up  by  successive 


Fig.  61.— Plan  of  the  Peak  of  Teneriffe,  showing  the  large  crater  and  minor  cones. 

outpourings  of  lava  from  different  sides,  and  by  showers  of 
dust  and  scoriae.  At  first,  the  lava,  if  the  sides  of  the  cone 
are  strong  enough  to  resist  its  pressure,  may  rise  until  it 
overflows  from  the  crater.  Subsequently,  as  the  funnel  be- 
comes choked  up,  and  the  cone  is  shattered  by  repeated 
explosions,  the  lava  finds  egress  from  different  fissures  and 
openings  on  the  cone.  As  the  mountain  increases  in  height, 
the  number  of  lava-currents  from  its  summit  will  usually 

120  B.  Reyer  (Jahrb.  Geol.  Reichs.  1879,  p.  463)  lias  experimentally  imitated 
the  process  of  extrusion  by  forcing  up  plaster  of  Paris  through  a  hole  in  a  board. 
For  drawings  of  the  Puy  de  Sarcouy  and  other  dome-shaped  hills  which  pre- 
sumably have  had  this  mode  of  origin,  see  Scrope's  "Geology  and  Extinct  Vol- 
canoes of  Central  France."  Refer  also  to  the  remarks  already  made  on  the 
liquidity  of  lava  (ante,  pp.  378-384),  and  the  account  of  "Vulkanische  Kup- 
pen, "  postea,  p.  433. 


422 


TEXT-BOOK    OF   GEOLOGY 


decrease.  Indeed,  the  taller  a  volcanic  cone  grows,  the  less 
frequently  as  a  rule  does  it  erupt.  The  lofty  volcanoes  of 
the  Andes  have  each  seldom  been  more  than  once  in  erup- 
tion during  a  century.  The  Peak  of  Teneriffe  (Fig.  61)  was 
three  times  active  during  370  years  prior  to  1798.181  The 


Pig.  62.— Map  of  Etna,  after  Sartorius  von  Waltershausen. 

1,  Lava  of  1879;  2,  Lavas  of  1865  and  1852;  3,  Lava  of  1669;  4,  Recent  Lavas;  6,  Lavas 

of  the  Middle  Ages;  6,  Ancient  Lavas  of  unknown  date;  7,  (.'ones  and  Craters; 

8,  Non-volcanic  Bocks. 

earlier  efforts  of  a  volcano  tend  to  increase  its  height,  as  well 
as  its  breadth;  the  later  eruptions  chiefly  augment  the 
breadth,  and  are  often  apt  to  diminish  the  height  by  blow- 
ing away  the  upper  part  of  the  cone.  The  formation  of 
fissures  and  the  consequent  intrusion  of  a  network  of  lava- 

181  For  a  recent  account  of  Teneriffe,  see  A.  Rothpletz,  Pctermann's  Mittlieil. 
xxxv.  (1889),  p.  237. 


DYNAMICAL    GEOLOGY 


dikes,  tend  to  bind  the  framework  of  the  volcano  and 
strengthen  it  against  subsequent  explo- 
sions. In  this  way,  a  kind  of  oscillation 
is  established  in  the  form  of  the  cone,  pe- 
riods of  crater-eruptions  being  succeeded 
by  others  when  the  emissions  take  place 
only  laterally  (ante,  p.  369). 

One  consequence  of  lateral  eruption  is 
the  formation  of  minor  parasitic  cones  on 
the  flanks  of  the  parent  volcano  (p.  329). 
Those  on  Etna,  more  than  200  in  number, 
are  really  miniature  volcanoes,  some  of 
them  reaching  a  height  of  700  feet  (Fig. 
62).  As  the  lateral  vents  successively  be- 
come extinct,  the  cones  are  buried  under 
sheets  of  lava  and  showers  of  debris  thrown 
out  from  younger  openings  or  from  the 
parent  cone.  It  sometimes  happens  that 
the  original  funnel  is  disused,  and.  that  the 
eruptions  of  the  volcano  take  place  from  a 
newer  main  vent.  Vesuvius,  for  example  <Sl 
(as  shown  in  Figs.  63  and  45),  stands  on 
the  site  of  a  portion  of  the  rim  of  the  more 
ancient  and  much  larger  vent  of  Monte 
Somma.  The  present  crater  of  Etna  lies 
to  the  northwest  of  the  former  vaster  crater. 
The  pretty  little  example  of  this  shifting 
furnished  by  Volcanello  has  been  already 
noticed  (p.  415). 

While,  therefore,  a  volcano,  and  more 
particularly  one  of  great  size,  throwing 
put  both  lava  and  fragmentary  materials, 
is  liable  to  continual  modification  of  its 
external  form,  as  the  result  of  successive 
eruptions,  its  contour  is  likewise  usually 
exposed  to  extensive  alteration  by  the 
effects  of  ordinary  atmospheric  erosion, 
as  well  as  from  the  condensation  of  the 
volcanic  vapors.  Heavy  and  sudden  floods, 
produced  by  the  rapid  rainfall  consequent 
upon  a  copious  discharge  of  steam,  rush 
down  the  slopes  with  such  volume  and  force 
as  to  cut  deep  gullies  in  the  loose  or  only 
partially  consolidated  tuffs  and  scoriaB.  Bain  continues  the 
erosion  until  the  outer  slopes,  unless  occasionally  renewed 


424:  TEXT-BOOK    OF   GEOLOGY 

by  fresh  showers  of  detritus,  assume  a  curiously  furrowed 
aspect,  like  a  half-opened  umbrella,  the  ridges  being  sepa- 
rated by  furrows  that  narrow  upward  toward  the  summit  of 
the  cone.  The  outer  declivities  of  Monte  Soinma  afford  an 
excellent  illustration  of  this  form  of  surface,  the  numerous 
ravines  on  that  side  of  the  mountain  presenting  instructive 
sections  of  the  prehistoric  lavas  and  tuns  of  the  earlier  and 
more  important  period  in  the  history  of  this  volcano.1" 
Similar  trenches  have  been  eroded  on  the  southern  or  Vesu- 
vian  side  of  the  original  cone,  but  these  have  in  great  meas- 
ure been  filled  up  by  the  lavas  of  the  younger  mountain. 
The  ravines,  in  fact,  form  natural  channels  for  the  lava,  as 
may  unfortunately  be  seen  round  the  Yesuvian  observatory. 
This  building  is  placed  on  one  of  the  ridges  between  two 
deep  ravines;  but  the  lava-streams  of  recent  years  have 
poured  into  these  ravines  on  either  side,  and  are  rapidly 
filling  them  up. 

Submarine  Volcanoes.— It  is  not  only  on  the  surface  of 
the  land  that  volcanic  action  shows  itself.  It  takes  place 
likewise  under  the  sea,  and  as  the  geological  records  of  the 
earth's  past  history  are  chiefly  marine  formations,  the  char- 
acteristics of  submarine  volcanic  action  have  no  small  inter- 
est for  the  geologist.  In  a  few  instances,  the  actual  out- 
break of  a  submarine  eruption  has  been  witnessed.  Thus, 
in  the  early  summer  of  1783,  a  volcanic  eruption  took  place 
about  thirty  miles  from  Cape  Reykjanaes  on  the  west  coast 
of  Iceland.  An  island  was  built  up,  from  which  fire  and 
smoke  continued  to  issue,  but  in  less  than  a  year  the  waves 
have  washed  the  loose  pumice  away,  leaving  a  submerged 
reef  from  five  to  thirty  fathoms  below  sea-level.  About  a 
month  after  this  eruption,  the  frightful  outbreak  of  Skaptar 
Jokull,  already  referred  to  (p.  378),  began,  the  distance  of 
this  mountain  from  the  submarine  vent  being  nearly  200 
miles.1**  A  century  afterward,  viz.  in  July,  1884,  another 


1M  See  H.  J.  Johnstoii-Lbivis,  Q.  J.  Geol.  Soc.  xl.  p.  103. 
183  Ljrell,  "Principles,"  ii.  p.  49. 


DYNAMICAL    GEOLOGY  425 

volcanic  island  is  said  to  have  been  thrown  up  near  the 
same  spot,  having  at  first  the  form  of  a  flattened  cone,  but 
soon  yielding  to  the  power  of  the  breakers.  Many  sub- 
marine eruptions  have  taken  place  within  historic  times 
in  the  Mediterranean.  The  most  noted  of  these  occurred 
in  the  year  1831,  when  a  new  volcanic  island  (Graham's 
Island,  He  Julia)  was  thrown  up,  with  abundant  discharge 
of  steam  and  showers  of  scoriae,  between  Sicily  and  the 


Fig.  64.— Sketch  of  submarine  volcanic  eruption  (Sabrina  Island)  off  St.  Michael's, 
June,  1811. 

coast  of  Africa.  It  reached  an  extreme  height  of  200  feet 
or  more  above  the  sea-level  (800  feet  above  sea-bottom) 
with  a  circumference  of  3  miles,  but  on  the  cessation  of. 
the  eruptions  was  attacked  by  the  waves  and  soon  demol- 
ished, leaving  only  a  shoal  to  mark  its  site.184  In  the  year 
1811,  another  island  was  formed  by  submarine  eruption  of 

124  Phil  Trans.  1832.  Constant  Prevost,  Ann.  des  Sci.  Nat.  xxiv.  Mem. 
Soc.  Geol.  France,  ii.  p.  91.  Mercalli's  "Vulcani,"  etc.,  p.  117.  For  a  recent 
submarine  eruption  in  the  Mediterranean,  see  Bicco,  Compt.  Rend.  Nov.  23, 
1891.' 


426  TEXT-BOOK   OF   GEOLOGY 

the  coast  off  St.  Michael's  in  the  Azores  (Fig.  64).  Con- 
sisting, like  the  Mediterranean  example,  of  loose  cinders, 
it  rose  to  a  height  of  about  300  feet,  with  a  circumference 
of  about  a  mile,  but  subsequently  disappeared.1"  In  the 
year  1796  the  island  of  Johanna  Bogoslawa,  in  Alaska,  ap- 
peared above  the  water,  and  in  four  years  had  grown  into 
a  large  volcanic  cone,  the  summit  of  which  was  3,000  feet 
above  sea-level.1" 

Unfortunately,  the  phenomena  of  recent  volcanic  erup- 
tions under  the  sea  are  for  the  most  part  inaccessible.  Here 
and  there,  as  in  the  Bay  of  Naples,  at  Etna,  among  the 
islands  of  the  Greek  Archipelago,  and  at  Tahiti,  elevation 
of  the  sea-bed  has  taken  place,  and  brought  to  the  surface 
beds  of  tuff  or  of  lava,  which  have  consolidated  under  water. 
Both  Vesuvius  and  Etna  began  their  career  as  submarine 
volcanoes.187  It  will  be  seen  from  the  accompanying  chart 
(Fig.  65),  that  the  Islands  of  Santorin  and  Therasia  form 
the  unsubmerged  portions  of  a  great  crater-rim  rising  round 
a  crater  which  descends  1278  feet  below  sea-level.  The 
materials  of  these  islands  consist  of  a  nucleus  of  marbles 
and  schists  nearly  buried  under  a  pile  of  tuft's  (trass), 
scoria?,  and  sheets  of  lava,  the  bedded  character  of  which 
is  well  shown  in  the  accompanying  sketch  by  Admiral 
Spratt  (Fig.  66),  who,  with  the  late  Prof.  Edward  Forbes, 
examined  the  geology  of  this  interesting  district  in  1841. 
They  found  some  of  the  tuffs  to  contain  marine  shells,  and 
thus  to  bear  witness  to  an  elevation  of  the  sea-floor  since 
volcanic  action  began.  More  recently  the  islands  have  been 
carefully  studied  by  various  observers.  K.  von  Fritsch  has 


l*  De  la  Becbe,  "Geological  Observer,"  p.  70. 

m  D.  Forbes,  Geol.  Mag.  vii.  p.  323. 
m  See,  aa  regards  Etna,  "Der  Aetna,"  ii.  p.  327. 


DYNAMICAL    GEOLOGY 


427 


found  recent  marine  shells  in 
many  places  up  to  heights  of 
nearly  600  feet  above  the  sea. 
The  strata  containing  these 
remains  he  estimates  to  be  at 
least  100  to  120  metres  thick, 
and  he  remarks  that  in  every 
case  he  found  them  to  consist 
essentially  of  volcanic  debris 


Fig.  85.— Map  of  partially-submerged  Tol« 

cano  of  Santorin. 

o,  Thera,  or  Santorin;  b,  Therasia;  c,  Mi- 
kro  Kaimeni;  d,  Neo  Kaimeni.  The  fig- 
ures denote  soundings  in  fathoms,  the 
dotted  line  marks  the  100  fathoms  line. 

and    to    rest    upon    volcanic 
rocks.      It  is   evident,   there- 
fore, that  these   shell-bearing 
tuffs    were    originally    de- 
posited   on    the    sea-floor 
after  volcanic   action   had 
begun  here,  and  that  dur- 
ing later  times  they  were 


^28  TEXT-BOOK   OF   GEOLOGY 

upraised,  together  with  the  submarine  lavas  associated 
with  them.1*8  Fouque'  concludes  that  the  volcano  formed 
at  one  time  a  large  island  with  wooded  slopes  and  a 
somewhat  civilized  human  population,  cultivating  a  fer- 
tile valley  in  the  southwestern  district,  and  that  in  prehis- 
toric times  the  tremendous  explosion  occurred  whereby  the 
centre  of  the  island  was  blown  out. 

The  similarity  of  the  structure  of  Santorin  to  that  of 
Somma  and  Etna  is  obvious.  Volcanic  action  still  contin- 
ues there,  though  on  a  diminished  scale.  In  1866-67  an 
eruption  took  place  on  Neo  Kaimeni,  one  of  the  later- 
formed  islets  in  the  centre  of  the  old  crater,  and  greatly 
added  to  its  area  and  height.  The  recent  eruptions  of  San- 
torin, which  have  been  studied  in  great  detail,  are  specially 
interesting  from  the  additional  information  they  have  sup- 
plied as  to  the  nature  of  volcanic  vapors  and  gases.  Among 
these,  as  already  stated  (p.  334),  free  hydrogen  plays  an 
important  part,  constituting,  at  the  focus  of  discharge,  thirty 
per  cent  of  the  whole.  By  their  eruption  under  water,  the 
mingling  of  these  gases  with  atmospheric  air  and  the  com- 
bustion of  the  inflammable  compounds  is  there  prevented, 
so  that  the  gaseous  discharges  can  be  collected  and  ana- 
lyzed. Probably  were  operations  of  this  kind  more  prac- 
ticable at  terrestrial  volcanoes,  free  hydrogen  and  its 
compounds  would  be  more  abundantly  detected  than  has 
hitherto  been  possible. 

The  numerous  volcanoes  which  dot  the  Pacific  Ocean, 


"8  See  Fritsch,  Z.  Deutsch.  Geol.  Ges.  xxiii.  (1871),  pp.  125-213.  The  most 
complete  and  elaborate  work  is  Fouque"'s  monograph  (already  cited),  "Santorin 
et  sea  Eruptions, "  Paris,  4to,  1880,  where  copious  analyses  of  rocks,  minerals 
and  gaseous  emanations,  with  maps  and  numerous  admirable  views  and  sections, 
are  given.  In  this  volume  a  bibliography  of  the  locality  will  be  found.  Corn- 
pare  C.  Doelter  on  Ihe  Ponza  Islands,  Denksch.  Akad.  Wissensch.  Vienna,  xxxvi. 
p.  141.  Sitz.  Akad.  "Wissensch.  Vienna,  Ixxi.  (1875),  p.  49. 


DYNAMICAL    GEOLOGY  429 

probably  in  most  cases  began  their  career  as  submarine 
vents,  their  eventual  appearance  as  subaerial  cones  being 
mainly  due  to  the  accumulation 
of  erupted  material,  but  also  par- 
tially, as  in  the  case  of  Santorin, 
to  actual  upheaval  of  the  sea- 
bottom.  The  lonely  island  of  St. 
Paul  (Figs.  67  and  69),  lying  in 
the  Indian  Ocean  more  than  2000 
miles  from  the  nearest  land  is  a 
notable  example  of  the  summit 

Fig.  67.— Volcanic  crater  of  St. 

of  a    volcanic    mountain   rising   to      paul  Island' Indian  ocean, 
the    sea-level    in    mid-ocean.      Its    circular  crater,   broken 
down   on   the   northeast  side,  is   filled    with  water   having 
a  depth   of  30  fathoms.11* 

Observations  by  K.  von  Drasche  have  shown  that  at 
Bourbon  (Reunion),  during  the  early  submarine  eruptions 
of  that  volcano,  coarsely  crystalline  rocks  (gabbro)  were 
emitted,  that  these  were  succeeded  by  andesitic  and  tra- 
chytic  lavas:  but  that  when  the  vent  rose  above  the  sea, 
basalts  were  poured  out.130  Fouque"  observes  that  at  San- 
torin some  of  the  early  submarine  lavas  are  identical  with 
those  of  later  subaerial  origin,  but  that  the  greater  part  of 


129  For  a  general  account  of  the  volcanic  islands  of  the  ocean,  see  Darwin's 
"Volcanic  Islands, "  2d  edit.  1876.     For  the  Philippine  volcanoes,  see  R.  von 
Drasche,  Tschermak's   Mirieralogische  Mittheil.   1876;    Semper's  "Die  Philip- 
pinen  und  ihre  Bewohner,"  Wurzburg,  1869.     For  the  Kurile  Islands,  J.  Milne, 
G-eol.  Mag.  1879,  1880,  1881 ;  Volcanoes  of  Bay  of  Bengal  (Barren  Island,  etc.), 
V.  Ball,  Geol.  Mag.  1879,  p.  16;  1888,  p.  404;  F.  R.  Mallet,  Mem.  G-eol.  Surv. 
India,  xxi.  part  iv.     St.  Paul  (Indian  Ocean),  C.  Velain,  Assoc.  Fran.  1875,  p. 
581;  "Mission  a  i'ile  St.  Paul."  1879;  "Description  geologique  de  la  Presqu'ile 
d'Aden,"  etc.,  4to,  Paris,  1878;  and  "Les  Voleans,"  1884.     For  Isle  of  Bour- 
bon, see  authorities  cited  on  p.  415,  and  for  Hawaii,  the  references  on  p.  350. 

130  Tschermak's  Mineralogische  Mittheil.  1876,  pp.  42,  157.     A  similar  struc- 
ture occurs  at  Palma  (Cohen,  Neues  Jahrb.  1879,  p.  482)  and  in  St.  Paul  (Velain 
as  above  cited). 


430  TEXT-BOOK    OF   GEOLOGY 

them  belong  to  an  entirely  different  series,  being  acid 
rocks,  belonging  to  the  group  of  hornblende-andesites, 
while  the  subaerial  rocks  are  augite-andesites.  The  acidity 
of  these  lavas  has  been  largely  increased  by  the  infusion 
into  them  of  much  silica,  chiefly  in  the  form  of  opal.  They 
differ  much  in  aspect,  being  sometimes  compact,  scoriaceous, 
hard,  like  millstone,  with  perlitic  and  spherulitic  structures, 
while  they  frequently  present  the  characters  of  trass  im- 
pregnated with  opal  and  zeolites.  Among  the  fragmental 
ejections  there  occur  blocks  of  schist  and  granitoid  rocks, 
probably  representing  the  materials  below  the  sea-floor 
through  which  the  first  explosion  took  place  (pp.  341,  363, 
417).  During  the  eruption  of  1866  some  islets  of  lava  rose 
above  the  sea  in  the  middle  of  the  bay,  near  the  active 
vent.  The  rock  in  these  cases  was  compact,  vitreous,  and 
much  cracked.131 

Among  submarine  volcanic  formations,  the  tuffs  differ 
from  those  laid  down  on  land  chiefly  in  their  organic  con- 
tents; but  partly  also  in  their  more  distinct  and  originally 
less  inclined  bedding,  and  in  their  tendency  to  the  ad- 
mixture of  non-volcanic  or  ordinary  mechanical  sediment 
with  the  volcanic  dust  and  stones.  No  appreciable  dif- 
ference either  in  external  aspect  or  in  internal  structure 
seems  yet  to  have  been  established  between  subaerial  and 
submarine  lavas.  Some  undoubtedly  submarine  lavas  are 
highly  scoriaceous.  There  is  no  reason,  indeed,  why  slaggy 
lava  and  loose,  non-buoyant  scoriae  should  not  accumulate 
under  the  pressure  of  a  deep  column  of  the  ocean.  At 
the  Hawaii  Islands,  on  25th  February,  1877,  masses  of 
pumice,  during  a  submarine  volcanic  explosion,  were 

131  Fouque,  "Santorin." 


DYNAMICAL    GEOLOGY 


431 


ejected  to  the  surface,  one  of  which  struck  the  bottom 
of  a  boat  with  considerable  violence  and  then  floated. 
When  we  reflect,  indeed,  to  what  a  considerable  extent 
the  bottom  of  the  great  ocean-basins  is  dotted  over  with 
volcanic  cones,  rising  often  solitary  from  profound  depths', 
we  can  believe  that  a  large  proportion  of  the  actual  erup- 
tions in  oceanic  areas  may  take  place  under  the  sea.  The 
immense  abundance  and  wide  diffusion  of  volcanic  detritus 
(including  blocks  of  pumice)  over  the  bottom  of  the  Pacific 


Fig.  68. -View  of  the  Peak  of  Teneriffe  and  its  coast-erosion. 

and  Atlantic  oceans,  even  at  distances  remote  from  land,  as 
made  known  by  the  voyage  of  the  "Challenger,"  doubtless 
indicate  the  prevalence  and  persistence  of  submarine  vol- 
canic action,  even  though,  at  the  same  time,  an  extensive 
diffusion  of  volcanic  debris  from  the  islands  is  admitted  to 
be  effected  by  winds  and  ocean -currents. 

Volcanic  islands,  unless  continually  augmented  by  re- 
newed eruptions,  are  attacked  by  the  waves  and  cut  down. 
Graham's  Island  and  the  other  examples  above  cited  show 


432  TEXT-BOOK    OF   GEOLOGY 

how  rapid  this  disappearance  may  be.  The  island  of  Vol- 
cano has  the  base  of  its  slopes  truncated  by  a  line  of  cliff 
due  to  marine  erosion.  The  island  of  Teneriffe  shows,  in 
the  same  way,  that  the  sea  is  cutting  back  the  land  toward 
the  great  cone  (Fig.  68).  The  island  of  St.  Paul  (Figs.  67, 
69)  brings  before  us  in  a  more  impressive  way  the  tendency 


Fig.  69.— View  of  St.  Paul  Island,  Indian  Ocean,  from  the  east  (Capt.  Blackwood 
in  Admiralty  Chart). 

a,  Nine-pin  Rock,  a  stack  of  harder  rock  left  by  the  sea;  6,  entrance  to  crater  lagoon 
(see  Fig.  67);  c,  d,  e,  cliffs  composed  of  bedded  volcanic  materials  dipping  toward 
the  south,  and  much  eroded  at  the  higher  end  (c)  by  waves  and  subaerial  waste ; 
/,  southern  point  of  the  island,  likewise  cut  away  into  a  cliff. 

of  volcanic  islands  to  be  destroyed  unless  replenished  by 
continual  additions  to  their  surface.  At  St.  Helena  lofty 
cliffs  of  volcanic  rocks  1000  to  2000  feet  high  bear  witness 
to  the  enormous  denudation  whereby  masses  of  basalt  two 
or  three  miles  long,  one  or  two  miles  broad,  and  1000 
to  2000  feet  thick,  have  been  entirely  removed.1" 

ii.   Fissure  (Massive)  Eruptions 

Under  the  head  of  massive  or  homogeneous  volcanoes 
some  geologists  have  included  a  great  number  of  bosses  or 
dome-like  projections  of  once-melted  rock  which,  in  regions 
of  extinct  volcanoes,  rise  cpnspicuously  above  the  surface 
without  any  visible  trace  of  cones  or  craters  of  fragmentary 
material.  They  are  usually  regarded  as  protrusions  of  lava, 
which,  like  the  Puy  de  Dome  in  Auvergne,  assumed  a 
dome-form  at  the  surface  without  spreading  out  in  sheets 

132  Darwin,  "Volcanic  Islands,"  p.  104.     For  a  more  detailed  account  of  this 
island,  see  J.  C.  Melliss'  "St.  Helena,"  London,  1875. 


DYNAMICAL    GEOLOGY  433 

over  the  surrounding  country,  and  with  no  accompanying 
fragmentary  discharges.  But  the  mere  absence  of  ashes  and 
scorisB  is  no  proof  that  these  did  not  once  exist,  or  that  the 
present  knob  or  boss  of  lava  may  not  originally  have  so- 
lidified within  a  cone  of  tuff  which  has  been  subsequently 
removed  in  denudation.  The  extent  to  which  the  surface 
of  the  ground  has  been  changed  by  ordinary  atmospheric 
waste,  and  the  comparative  ease  with  which  loose  volcanic 
dust  and  cinders  might  have  been  entirely  removed,  require 
to  be  considered.  Hence,  though  the  ordinary  explanation 
is  no  doubt  in  some  cases  correct,  it  may  be  doubted 
whether  a  large  proportion  of  the  examples  cited  from  the 
Rhine,  Bohemia,  Hungary,  and  other  regions,  ought  not 
rather  to  be  regarded,  like  the  "necks"  so  abundant  in  the 
ancient  volcanic  districts  of  Britain  (Book  IV.  Part  VII.), 
as  the  remaining  roots  of  ordinary  volcanic  cones.  If  the 
tuff  of  a  cone,  up  the  funnel  of  which  lava  rose  and  solidi- 
fied, were  swept  away,  we  should  find  a  central  lava  plug 
or  core  resembling  the  volcanic  "heads"  (vulkanische  Kup- 
peri)  of  Germany.  Unquestionably,  lava  has  in  innumer- 
able instances  risen  in  this  way  within  cones  of  tuff  or 
cinders,  partially  filling  them  without  flowing  out  into  the 
surrounding  country.183 

But  while,  on  either  explanation  of  their  origin,  these 
volcanic  "heads"  find  their  analogues  in  the  emissions  of 
lava  in  modern  volcanoes,  there  are  numerous  cases  in  old 
volcanic  areas  where  the  eruptions,  so  far  as  can  now  be 
judged,  were  not  attended  with  the  production  of  any 
central  cone  or  crater.  Such  emissions  of  lava  may  have 


i«s  yon  geebach,  Z.  Deutsch.  Geol.  Ges.  xviii.  p.  643.  F.  von  Hochstetter, 
Neues  Jahrb.  1871,  p.  469.  Reyer,  Jahrb.  K.  K.  Geol.  Reichsanstalt,  1878, 
p.  81;  1879,  p.  463. 

GEOLOGY— Vol.  XXIX— 19 


484  TEXT-BOOK    OF   GEOLOGY 

resembled  those  which  in  recent  times  have  occurred  at  the 
Hawaiian  volcanoes,  where  enormous  accumulations  of  lava 
have  gradually  been  built  up  into  flat  domes,  of  which 
Mauna  Loa  rises  to  a  height  of  13,675  feet.  Vast  floods  of 
remarkably  liquid  basic  lava  have  from  time  to  time  flowed 
out  tranquilly  without  explosion  or  earthquake,  and  with 
no  accompaniment  of  fragmental  discharges.  These  cur- 
rents of  molten  rock  have  spread  out  into  wide  sheets, 
sloping  at  so  low  an  angle  that  they  look  horizontal.  The 
lower  and  older  portions  of  them  have  been  eroded  by 
streams  so  as  to  present  escarpments  and  outliers  not  un- 
like those  of  western  North  America  or  the  older  basaltic 
plateaus  of  Britain  and  India.1'4 

The  most  stupendous  modern  basaltic-floods  of  Iceland 
issued  from  vents  along  a  fissure.  According  to  Thorodd- 
sen  the  post-glacial  lava-fields  of  Odadahraun,  covering 
an  area  of  about  4390  square  kilometres,  have  issued  from 
about  20  distinct  vents,  while  in  the  east  of  Iceland  the 
lava  has  flowed  from  the  lips  of  fissures.186  It  would  seem 
that  for  the  discharge  of  such  wide  and  flat  sheets  of  lava, 
great  mobility  and  tolerably  complete  fusion  of  the  molten 
mass  is  necessary.  The  phenomenon  occurs  among  the 
more  basic  lavas  (basalts,  etc.)  rather  than  among  the  more 
lithoid  acid  lavas  (trachytes,  rhyolites,  etc.). 

In  former  geological  ages,  extensive  eruptions  of  lava, 
without  the  accompaniment  of  scoriae,  with  hardly  any 
fragmentary  materials,  and  with,  at  the  most,  only  flat 


134  For  a  graphic  account  of  the  Hawaiian  lava-fields,  see  Captain  Button, 
Fourth  Annual  Report,   U.   S.    Geol.    Survey  for  1882-83.     See  also  Dana's 
"Characteristics  of  Volcanoes." 

135  See  W.   L.  Watts'  "Across  the  Vatna  Jokull,"  Proc.  Roy.  Geog.  Soc. 
1876.     W.  G.  Lock,  Geol.  Mag.  1881,  p.  212;  and  papers  by  Thoroddsen  and 
Holland,  quoted  ante,  p.  345. 


DYNAMICAL    GEOLOGY  435 

dome-shaped  cones  at  the  points  of  emission,  have  taken 
place  over  wide  areas  from  scattered  vents,  along  lines  or 
systems  of  fissures.  Vast  sheets  of  lava  have  in  this  man- 
ner been  poured  out  to  a  depth  of  many  hundred  feet,  com- 
pletely burying  the  previous  surface  of  the  land  and  forming 
wide  plains  or  plateaus.  These  truly  "massive  eruptions" 
have  been  held  by  Bichthofen136  and  others  to  represent  the 
grand  fundamental  character  of  volcanisrn,  ordinary  vol- 
canic cones  being  regarded  merely  as  parasitic  excrescences 
on  the  subterranean  lava-reservoirs,  .very  much  in  the  rela- 
tion of  minor  cinder  cones  to  their  parent  volcano.1" 

Though  a  description  of  these  old  fissure  or  massive 
eruptions  ought  properly  to  be  included  in  Book  IV.,  the 
subject  is  so  closely  connected  with  the  dynamics  of  exist- 
ing active  volcanoes  that  an  account  of  the  subject  may  be 
given  here.  Perhaps  the  most  stupendous  example  of  this 
type  of  volcanic  structure  occurs  in  Western  North  America. 
The  extent  of  country  which  has  been  flooded  with  basalt  in 
Oregon,  Washington,  California,  Idaho,  and  Montana  has 
not  yet  been  accurately  surveyed,  but  has  been  estimated 
to  cover  a  larger  area  than  France  and  Great  Britain  com- 
bined, with  a  thickness  averaging  2000  but  reaching  in  some 
places  to  3700  feet.138  The  Snake  River  plain  in  Idaho  (Fig. 
70)  forms  part  of  this  lava-flood.  Surrounded  on  the  north 
and  east  by  lofty  mountains,  it  stretches  westward  as  an  ap- 
parently boundless  desert  of  sand  and  bare  sheets  of  black 
basalt.  A  few  streams  descending  into  the  plain  from  the 
lulls  are  soon  swallowed  up  and  lost.  The  Snake  River, 
however,  flows  across  it,  and  has  cut  out  of  its  lava-beds  a 


138  Trans.  Akad.  Sci.  California,  1868. 

137  Proc.  Roy.  Phys.  Soc.  Edin.  v.  236;  Nature,  xxiii.  p.  3. 

138  J.  LeConte.     Amer.  Journ.  Sci.  3d  ser.  vii.  (1874),  167,  259. 


436 


TEXT-BOOK    OF   GEOLOGY 


series  of  picturesque  gorges  and  rapids.  Looked  at  from 
any  point  on  its  surface,  it  appears  as  a  vast  level  plain 
like  that  of  a  lake-bottom,  though  more  detailed  examina- 
tion may  detect  a  slope  in  one  or  more  directions,  and  may 
thereby  obtain  evidence  as  to  the  sites  of  the  chief  openings 
from  which  the  basalt  was  poured  forth.  The  uniformity 
of  surface  has  been  produced  either  by  the  lava  flowing 
over  a  plain  or  lake-bottom,  or  by  the  complete  effacement 
of  an  original  and  undulating  contour  of  the  ground  under 


Fig.  70.— View  of  the  great  Basalt-plain  of  the  Snake  River,  Idaho,  with  recent  cones. 

hundreds  of  feet  of  volcanic  rock  in  successive  sheets.  The 
lava  rolling  up  to  the  base  of  the  mountains  has  followed 
the  sinuosities  of  their  margin,  as  the  waters  of  a  lake  follow 
its  promontories  and  bays.  The  author  crossed  the  Snake 
River  plain  in  1879,  and  likewise  rode  for  many  miles  along 
its  northern  edge.  He  found  the  surface  to  be  everywhere 
marked  with  low  hummocks  or  ridges  of  bare  black  basalt, 
the  surfaces  of  which  exhibited  a  reticulated  pavement  of 
the  ends  of  columns.  In  some  places,  there  was  a  percep- 


DYNAMICAL  GEOLOGY  437 

tible  tendency  in  these  ridges  to  range  themselves  in  one 
general  northeasterly  direction,  when  they  might  be  likened 
to  a  series  of  long,  low  waves,  or  ground-swells.  In  many 
instances  the  crest  of  each  ridge  had  cracked  open  into  a 
fissure  which  presented  along  its  walls  a  series  of  tolerably 
symmetrical  columns  (Fig.  70).  That  these  ridges  were 
original  undulations  of  the  lava,  and  had  not  been  pro- 
duced by  erosion,  was  indicated  by  the  fact  that  the  col- 
umns were  perpendicular  to  their  surface,  and  changed 
in  direction  according  to  the  form  of  the  ground  which 
was  the  original  cooling  surface  of  the  lava.  Though  the 
basalt  was  sometimes  vesicular,  no  layers  of  slag  or  scoriae 
were  anywhere  observed,  nor  did  the  surfaces  of  the  ridges 
exhibit  any  specially  scoriform  character. 

There  are  no  great  cones  whence  this  enormous  flood 
of  basalt  could  have  flowed.  It  probably  escaped  from 
orifices  or  fissures  still  concealed  under  the  sheets  which 
issued  from  them,  the  points  of  escape  being  marked  only 
by  such  low  domes  as  could  readily  be  buried  under  the  suc- 
ceeding eruptions  from  other  vents.189  That  it  was  not  the 
result  of  one  sudden  outpouring  of  rock  is  shown  by  the  dis- 
tinct bedding  of  the  basalt,  which  is  well  marked  along  the 
river  ravines.  It  arose  from  what  may  have  been,  on  the 
whole,  a  continuous  though  locally  intermittent  welling-out 
of  lava,  probably  from  vents  on  many  fissures  extending 
over  a  wide  tract  of  Western  America  during  a  late  Tertiary 
period,  if,  indeed,  the  eruptions  did  not  partly  come  within 
the  time  of  the  human  occupation  of  the  continent.  The 
discharge  of  lava  continued  until  the  previous  topography 
was  buried  under  some  2000  feet  of  lava,  only  the  higher 

139  Captain  Dutton  has  remarked  the  absence  of  any  conspicuous  feature  at 
the  sources  from  which  some  of  the  largest  lava-streams  of  Hawaii  have  issued. 


438  TEXT-BOOK   OF   GEOLOGY 

summits  still  projecting  above  the  volcanic  flood.140  At  a 
few  points  on  the  plain  and  on  its  northern  margin,  the 
author  observed  some  small  cinder  cones  (Fig.  70).  These 
were  evidently  formed  during  the  closing  stages  of  volcanic 
action,  and  may  be  compared  to  the  minor  cones  on  a  modern 
volcano,  or  better,  to  those  on  the  surface  of  a  recent  lava- 
stream. 

In  Europe,  daring  older  Tertiary  time,  similar  enormous 
outpourings  of  basalt  covered  many  hundreds  of  square 
miles.  The  most  important  of  these  is  that  which  occupies 
a  large  part  of  the  northeast  of  Ireland,  and  in  disconnected 
areas  extends  through  the  Inner  Hebrides  and  the  Faroe 
Islands  into  Iceland.  Throughout  that  region,  the  paucity 
of  evidence  of  volcanic  vents  is  truly  remarkable.  So  ex- 
tensive has  been  the  denudation,  that  the  inner  structure  of 
the  volcanic  plateaus  has  been  admirably  revealed.  The 
ground  beneath  and  around  the  basalt-sheets  has  been  rent 
into  innumerable  fissures  which  have  been  filled  by  the  rise 
of  basalt  into  them.  A  vast  number  of  basalt-dikes  ranges 
from  the  volcanic  area  eastward  across  Scotland  and  the 
north  of  England  and  the  north  of  Ireland.  Toward  the 
west  the  molten  rock  reached  the  surface  and  was  poured 
out  there,  while  to  the  eastward  it  does  not  appear  to  have 
overflowed,  or,  at  least,  all  evidence  of  the  outflow  has  been 
removed  in  denudation.  When  we  reflect  that  this  system 
of  dikes  can  be  traced  from  the  Orkney  Islands  southward 
into  Yorkshire  and  across  Britain  from  sea  to  sea,  over  a 
total  area  of  probably  not  less  than  100,000  square  miles, 
we  can  in  some  measure  appreciate  the  volume  of  molten 
basalt  which  in  older  Tertiary  times  underlay  large  tracts  of 

140  Prof.  J.  LeGonte  believes  that  the  chief  fissures  opened  in  the  Cascade 
and  Blue  Mountain  Ranges.  Amer.  Journ.  Sci.  3d  series,  v.  (1874),  p.  168. 


DYNAMICAL    GEOLOGY  439 

the  site  of  the  British  Islands,  rose  up  in  so  many  thousands 
of  fissures,  and  poured  forth  at  the  surface  over  so  wide  an 
area  in  the  northwest.141 

In  Africa,  basaltic  plateaus  cover  large  tracts  of  Abys- 
sinia, where  by  the  denuding  effect  of  heavy  rains  they  have 
been  carved  into  picturesque  hills,  valleys,  and  ravines.14* 
In  India,  an  area  of  at  least  200,000  square  miles  is  covered 
by  the  singularly  horizontal  volcanic  plateaus  of  the  ."Dec- 
can  Traps"  (lavas  and  tuffs),  which  belong  to  the  Cretaceous 
period  and  attain  a  thickness  of  6000  feet  or  more.14'  The 
underlying  platform  of  older  rock,  where  it  emerges  from 
beneath  the  edges  of  the  basalt  table-land,  is  found  to  be  in 
many  places  traversed  by  dikes;  but  no  cones  and  craters 
are  anywhere  visible.  In  these,  and  probably  in  many  other 
examples  still  undescribed,  the  formation  of  great  plains  or 
plateaus  of  level  sheets  of  lava  is  to  be  explained  by  "fis- 
sure-eruptions" rather  than  by  the  operations  of  volcanoes 
of  the  familiar  "cone  and  crater"  type. 

§4.  Geographical   and   geological   distri- 
bution  of   volcanoes 

Adequately  to  trace  the  distribution  of  volcanic  actiom 
over  the  globe,  account  ought  to  be  taken  of  dormant  and 
extinct  volcanoes,  likewise  of  the  proofs  of  volcanic  out- 
breaks during  earlier  geological  periods.  When  this  is 
done,  we  learn,  on  the  one  hand,  that  innumerable  dis- 
tricts have  been  the  scene  of  prolonged  volcanic  activity, 
where  there  is  now  no  underground  commotion,  and  on  the 
other,  that  volcanic  outbursts  have  been  apt  to  take  place 

141  Trans.  Roy.  Soc.  Edin.  xxxv.  (1888),  p.  21. 

I4S  Blanford's  "Abyssinia,"  1870,  p.  181. 

143  Medlicott  and  Blanford,  "Geology  of  India,"  p.  299. 


440  TEXT-BOOK   OF   GEOLOGY 

again  and  again  after  wide  intervals  on  the  same  ground, 
some  modern  active  volcanoes  being  thus  the  descendants 
and  representatives  of  older  ones.  Some  of  the  facts  re- 
garding former  volcanic  action  have  been  already  stated. 
Others  will  be  given  in  Book  IV.  Part  VII. 

Confining  attention  to  vents  now  active,  of  which  the 
total  number  may  be  about  300, 144  the  chief  facts  regarding 
their  distribution  over  the  globe  may  be  thus  summarized. 
(1)  Volcanoes  occur  along  the  margins  of  the  ocean-basins, 
particularly  along  lines  of  dominant  mountain-ranges,  which 
either  form  part  of  the  mainland  of  the  continents  or  extend 
as  adjacent  lines  of  islands.  The  vast  hollow  of  the  Pacific 
is  girdled  with  a  wide  ring  of  volcanic  foci.  (2)  Volcanoes 
rise,  as  a  striking  feature,  from  the  submarine  ridges  that 
traverse  the  ocean  basins.  All  the  oceanic  islands  are  either 
volcanic  or  formed  of  coral,  and  the  scattered  coral-islands 
have  in  all  likelihood  been  built  upon  the  tops  of  submarine 
volcanic  cones.  (3)  Volcanoes  are  situated  not  far  from  the 
sea.  The  only  exceptions  to  this  rule  are  certain  vents  in 
Mantchuria  and  in  the  tract  lying  between  Thibet  and  Si- 
beria; but  of  the  actual  nature  of  these  vents  very  little  is 
yet  known.  (4)  The  dominant  arrangement  of  volcanoes 
is  in  series  along  subterranean  lines  of  weakness,  as  in  the 
chain  of  the  Andes,  the  Aleutian  Islands,  and  the  Malay 
Archipelago.  A  remarkable  zone  of  volcanic  vents  girdles 
the  globe  from  Central  A.merica  eastward  by  the  Azores  and 
Canary  Islands  to  the  Mediterranean,  thence  to  the  Red  Sea, 

144  This  number  is  probably  below  the  truth.  Prof.  J.  Milne  has  enumerated 
in  Japan  alone  no  fewer  than  fifty-three  volcanoes  which  are  either  active  or 
have  been  active  within  a  recent  period.  He  remarks  that,  "if  we  were  in  a 
position  to  indicate  the  volcanoes  which  had  been  in  eruption  during  the  last 
4,000  years,  the  probability  is  that  they  would  number  several  thousands  rather 
than  four  or  five  hundred."  "Earthquakes  and  other  Earth-movements,"  1886, 
p.  227.  Compare  Fisher,  "Physics  of  Earth's  Crust,"  2d  ed.  chap.  xxiv. 


DYNAMICAL    GEOLOGY  441 

and  through  the  chains  of  islands  from  the  south  of  Asia  to 
New  Zealand  and  the  heart  of  the  Pacific.  (5)  On  a  smaller 
scale  the  linear  arrangement  gives  place  to  one  in  groups, 
as  in  Italy,  Iceland,  and  the  volcanic  islands  of  the  great 
oceans. 

In  the  European  area  there  are  six  active  volcanoes — 
Vesuvius,  Etna,  Stromboli,  Volcano,  Santorin,  and  Nisyros. 
Asia  contains  twenty-four,  Africa  ten,  North  America 
twenty,1"  Central  America  twenty-five,  and  South  Amer- 
ica thirty-seven.14*  By  much  the  larger  number,  however, 
occur  on  islands  in  the  ocean.  In  the  Arctic  Ocean  rises  the 
solitary  Jan  Mayen.  On  the  ridge  separating  the  Arctic 
and  Atlantic  basins,  the  group  of  Icelandic  volcanoes  is 
found.  Along  the  great  central  ridge  of  the  Atlantic  bot- 
tom, numerous  volcanic  vents  have  risen  above  the  surface 
of  the  sea— the  Azores,  Canary  Islands,  and  the  extinct  de- 
graded volcanoes  of  St.  Helena,  Ascension  and  Tristan 
d'Acunha.  On  the  eastern  border  lie  the  volcanic  vents 
of  the  islands  off  the  African  coast,  and  to  the  west  those  of 
the  West  India  Islands.  Still  more  remarkable  is  the  de- 
velopment of  volcanic  energy  in  the  Pacific  area.  From  the 
Aleutian  Islands  southward,  a  long  line  of  volcanoes,  num- 
bering upward  of  a  hundred  active  vents,  extends  through 
Kamtschatka  and  the  Kurile  Islands  to  Japan,147  whence 


145  For  an  account  of  the  remarkable  extinct  volcanoes  of  Northern  Califor- 
nia, Oregon  and  Washington  Territory,  see  A.  Hague  and  J.  P.  Iddings.  Amer. 
Joura.  Sci.  xiYi.  (1883),  p.  222.  On  Volcanoes  of  Mexico  see  H.  Lenk,  "Bei- 
trage  zur  Geologic  und  Palseontologie  der  Republik  Mexico,"  Leipzig,  1890;  of 
Central  America,  A.  Dolfuss  and  E.  de  Monserrat,  "  Voyage  Geologique, ' '  Paris, 
folio,  1868;  K.  ron  Seeback,  Abh.  Kon.  Ges.  Wiss.  Gottingen,  xxxviii.  (1892). 

141  For  a  recent  account  of  the  volcanoes  of  the  Andes  of  the  Equator  see 
Whymper's  "Travels  amongst  the  Great  Andes." 

141  For  the  volcanoes  of  Japan,  besides  papers  quoted  on  p.  364,  see  W.  J. 
Holland,  Appalachia,  vi.  (1890),  109.  E.  Naumann,  Zeitsch.  Deutsch.  Geol. 
Ges.  1877,  p.  364.  Mr.  Milne  enumerates  100  active  vents  from  the  Kuriles  t« 
Kinshu  (2,000  miles). 


442  TEXT-BOOK    OF   GEOLOGY 

another  numerous  series  carries  the  volcanic  band  far  south 
toward  the  Malay  Archipelago,  which  must  be  regarded  as 
the  chief  centre  of  the  present  volcanic  activity  of  our  planet. 
In  Sumatra,  Java,  and  adjoining  islands,  no  fewer  than  fifty 
active  vents  occur.  The  chain  is  continued  through  New 
Guinea  and  the  groups  of  islands  to  New  Zealand.148  Even 
in  the  Antarctic  regions,  Mounts  Erebus  and  Terror  are 
cited  as  active  vents;  while  in  the  centre  of  the  Pacific 
Ocean  rise  the  great  lava  cones  of  the  Sandwich  Islands.  la 
the  Indian  Ocean,  the  Bed  Sea,  and  off  the  east  coast  of 
Africa  a  few  scattered  vents  appear. 

Besides  the  existence  of  extinct  volcanoes  which  have 
obviously  been  active  in  comparatively  recent  times,  the 
geologist  can  adduce  proofs  of  the  former  presence  of  active 
volcanoes  in  many  countries  where  cones,  craters,  and  all 
the  ordinary  aspects  of  volcanic  mountains,  have  long  dis- 
appeared, but  where  sheets  of  lava,  beds  of  tuff,  dikes,  and 
necks  representing  the  sites  of  volcanic  vents  have  been  rec- 
ognized abundantly  (Book  IV.  Part  VII.).  These  manifes- 
tations of  volcanic  action,  moreover,  have  as  wide  a  range 
in  geological  time  as  they  have  in  geographical  area.  Every 
great  geological  period,  back  into  pre-Cambrian  time,  seems 
to  have  had  its  volcanoes.  In  Britain,  for  instance,  there 
were  probably  active  volcanic  vents  in  pre-Cambrian  ages. 
The  Archaean  gneiss  of  N.  W.  Scotland  includes  a  remark- 
able series  of  dikes  presenting  some  points  of  resemblance 
to  the  great  Tertiary  system.  The  Torridon  sandstone  of 
the  same  region,  which  is  now  known  to  be  pre-Cambrian, 


148  The  great  eruption  of  Tarawera,  New  Zealand,  in  1886,  is  described  by 
Prof.  A.  P.  W.  Thomas,  "Report  on  the  Eruption  of  Tarawera,"  published 
by  the  Government  in  1888:  also  Prof.  Button's  "Report  on  the  Tarawera 
Tolcanic  District,  Wellington,  1887,"  Quart.  Journ.  Geol.  Soc.  xliii.  (1887), 
p.  178. 


DYNAMICAL    GEOLOGY  443 

contains  pebbles  of  various  finely  vesicular  porphyrites,  and 
in  one  place  includes  a  band  of  true  tuff.  In  the  lower  Cam- 
brian period  the  tuffs  and  diabases  of  Pembrokeshire  were 
erupted.  Still  more  vigorous  were  the  volcanoes  in  the 
Lower  Silurian  period,  when  the  lavas  and  tuffs  of  Snow- 
don,  Aran  Mowddwy,  and  Cader  Idris  were  ejected.  Dur- 
ing the  deposition  of  the  Upper  Silurian  rocks  a  few  vol- 
canoes were  active  in  the  west  of  Ireland.  The  Lower  Old 
Red  Sandstone  epoch  was  one  of  prolonged  activity  in  cen- 
tral Scotland.  The  earlier  half  of  the  Carboniferous  period 
likewise  witnessed  two  distinct  epochs  of  volcanic  activity 
over  the  same  region.  In  the  earlier  of  those,  lavas  (ande- 
Bites  and  trachytes)  were  poured  out  in  wide  level  plateaus 
from  many  vents,  while  in  the  later,  groups  of  minor  cones 
like  the  puys  of  Auvergne  were  dispersed  among  the  lagoons. 
During  Permian  time,  more  than  a  hundred  small  vents  rose 
in  scattered  groups  across  the  centre  and  southwest  of  Scot- 
land, while  a  few  similar  points  of  eruption  appeared  in  the 
southwest  of  England.  No  trace  of  any  British  Mesozoic 
volcanoes  has  been  met  with.  The  vast  interval  between 
Permian  and  older  Tertiary  time  appears  to  have  been  a 
period  of  total  quiescence  of  volcanic  activity.  The  older 
Tertiary  ages  were  distinguished  by  the  outpouring  of  the 
enormous  basaltic  plateaus  of  Antrim  and  the  Inner  Heb- 
rides.>4t 

In  France  and  Germany,  likewise,  Palaeozoic  time  was 
marked  by  the  eruption  of  many  diabase,  porphyrite,  and 
quartz-porphyry  lavas.  In  Brittany,  for  example,  Dr.  Bar- 
rois  has  found  a  remarkable  series  of  older  Palaeozoic  dia- 


149  For  a  detailed  summary  of  the  volcanic  history  of  Britain,  see  Presidential 
addresses  to  the  Geological  Society,  Quart.  Journ.  Geol.  Soc.  xlvii.,  xlviii. 
(1891-92). 


444  TEXT-BOOK   OF   GEOLOGY 

bases  and  porphyrites  with  tuft's  and  agglomerates.  He  dis- 
tinguishes four  principal  periods  of  eruption — 1.  Cambrian 
and  Lower  Silurian;  2.  Middle  and  Upper  Silurian ;  3.  Up- 
per Devonian;  4.  Carboniferous.160  The  Permian  period 
was  marked  in  G-ermany  and  also  in  the  south  of  France 
by  the  discharge  of  great  masses  of  various  quartz-porphy- 
ries. The  Triassic  period  likewise  witnessed  numerous  erup- 
tions. But  from  that  period  onward  the  same  remarkable 
quiescence  appears  to  have  reigned  all  over  Europe  which 
characterized  the  geological  history  of  Britain  during  Meso- 
zoic  time.161  In  Tertiary  time  a  prodigious  outpouring  of 
lavas,  both  acid  and  basic,  continued  from  the  Miocene 
epoch  down  even  perhaps  to  the  historic  period.  Examples 
of  this  great  series  are  met  with  in  Central  France,  the  Eifel, 
Italy,162  Bohemia,  and  Hungary,  almost  to  the  existing 
period.163  Recent  research  has  brought  to  light  evidence  of 
a  long  succession  of  Tertiary  and  post-Tertiary  volcanic  out- 
bursts in  Western  America  (Nevada,  Oregon,  Idaho,  Utah, 
etc.).  Contemporaneous  volcanic  rocks  are  associated  with 
Palaeozoic,  Secondary,  and  Tertiary  formations  in  New  Zea- 
land, and  volcanic  action  there  is  not  yet  extinct. 

Thus  it  can  be  shown  that,  within  the  same  compara- 
tively limited  geographical  space,  volcanic  action  has  been 

150  Carte  Geol.  detaill.  Prance,  No.  7,  T889. 

161  Some  trifling  exceptions  to  this  general  statement  are  said  to  occur. 
C.  E.  M.  Eohrbach  describes  Cretaceous  teschenites  and  diabases  in  Silesia 
(Tschermak's  Min.  Mittheil.  vii.  (1885,  p.  15).  P.  Choffat  refers  to  Cenoma- 
nian  eruptions  in  Portugal  (Journ.  Sciencias  Math.  Phys.  Natur,  Lisbon,  1884). 
A.  B.  Lagorio  has  found  in  the  Crimea  a  series  of  sheets,  dikes  and  bosses, 
ranging  from  nevadites  to  basalts. 

168  For  early  and  classical  accounts  of  the  Italian  volcanic  districts,  see  Spal- 
lanzani's  "Voyages  dans  les  deux  Siciles,"  and  Breislak's  "Voyages  Physiques 
et  Lythologiques  dans  la  Campanie."  Consult  also  Mercalli's  "Vulcani,"  etc., 
and  Johnston-Lavis'  "South  Italian  Volcanoes,"  already  cited. 

168  For  a  recent  attempt  to  give  a  stratigraphical  and  geographical  view  of 
the  distribution  of  igneous  rocks  in  Europe,  see  M.  Bertrand,  Bull.  Soc.  Geol. 
France,  xvi.  (1888),  p.  573. 


DYNAMICAL    GEOLOGY  446 

rife  at  intervals  during  a  long  succession  of  geological  ages. 
Even  round  the  sites  of  still  active  vents,  traces  of  far  older 
eruptions  may  be  detected,  as  in  the  case  of  the  existing  ac- 
tive volcanoes  of  Iceland,  which  rise  from  amid  Tertiary 
lavas  and  tuffs.  Volcanic  action,  which  now  manifests  itself 
so  conspicuously  along  certain  lines,  seems  to  have  contin- 
ued in  that  linear  development  for  protracted  periods  of 
time.  The  actual  vents  have  changed,  dying  in  one  place 
and  breaking  out  in  another,  yet  keeping  on  the  whole  along 
the  same  tracts.  Taking  all  the  manifestations  of  volcanic 
action  together,  both  modern  and  ancient,  we  see  that  the 
subterranean  forces  have  operated  along  great  lines  in  the 
earth's  crust,  and  that  the  existing  volcanoes  form  but  a 
small  proportion  of  the  total  number  of  once  active  vents. 

Looking  broadly  at  the  geological  history  of  volcanic  ac- 
tion we  observe  that,  while  there  is  evidence  of  the  protru- 
sion of  both  acid  and  basic  materials  from  the  remotest 
periods,  the  earlier  discharges  were  preponderantly  acid.  In 
Britain,  for  example,  the  vast  piles  of  lavas  ejected  during 
the  Silurian  period  were  mainly  of  a  felsitic  character,  though 
considerable  accumulations  of  andesites  were  not  wanting.  On 
the  other  hand,  the  wide  sheets  of  lava  poured  out  in  this 
country  during  Tertiary  time  were  chiefly  basalts,  the  acid 
protrusions  occurring  mostly  as  dikes  and  bosses.  A  similar 
broad  sequence  has  been  observed  in  other  countries. 

When,  however,  we  proceed  to  consider  more  closely  the 
nature  of  the  successive  eruptions  during  the  continuance  of 
one  of  the  volcanic  periods  of  which  records  are  preserved 
among  the  geological  formations,  we  discover  proofs  of  a  re- 
markable variation  in  the  character  of  the  lavas.184  Various 

154  In  some  volcanoes  (e.g.  Tenerifie)  the  lower  lavas  are  heavier  and  more 
basic  than  the  upper. 


446  TEXT-BOOK    OF   GEOLOGY 

observers  have  noticed  that  volcanic  rocks  have  succeeded 
each  other  in  a  certain  order  in  different  regions.  Baron 
von  Richthofen  deduced  from  observations  in  Europe  and 
America  a  general  sequence  of  volcanic  succession,  which 
he  arranged  in  the  following  order:  1.  Propylite;  2.  Ande- 
site;  3.  Trachyte;  4.  Rhyolite;  5.  Basalt.1"  This  sequence 
he  believed  to  be  seldom  or  never  complete  in  any  one  local- 
ity; sometimes  only  one  member  of  the  series  may  be  found; 
but  when  two  or  more  occur  they  follow,  in  his  opinion,  this 
sequence,  basalt  being  everywhere  the  latest  of  the  series. 
The  subject  has  been  more  recently  discussed  by  M.  Ber- 
trand,  who  remarks  that  in  Europe  each  of  the  great  areas 
of  plication  has  given  rise  to  the  formation  of  eruptive  rocks 
of  every  composition  and  structure.  He  recognizes  a  recur- 
rence of  the  phenomena  in  successive  geological  periods,  and 
speaks  of  a  definite  order  of  eruptions  in  the  same  series.16' 
The  great  volcanic  series  of  Auvergne  presents  a  marvel- 
lous succession  of  varied  eruptions  within  a  limited  region 
during  what  was  probably  a  single  volcanic  period.  The 
first  eruptions  appear  to  have  been  basalts,  and  rocks  of 
similar  character  reappeared  again  and  again  in  later  stages 
of  the  history,  the  intervening  eruptions  consisting  of  phono- 
lites,  trachytes,  rhyolites,  or  andesites.  The  latest  lavas 
were  scoriaceous  basalts.1"  Among  the  later  Palaeozoic  vol- 
canic eruptions  of  Britain  a  more  definite  and  regular  recur- 
rence of  rocks  appears  to  be  traceable.  The  earlier  lavas  of 
the  Old  Bed  Sandstone  and  Carboniferous  series  were  gen- 
erally either  intermediate  or  basic,  sometimes  remarkably 
basic,  while  the  late  protrusions  were  decidedly  acid.  At 


155  "The  Natural  System  of  Volcanic  Rocks,"  Californ.  Acad.  Sci.  1868. 

156  Bull.  Soc.  Geol.  France,  xvi.  (1888),  p.  611. 

151  Carte  Geol.  detail!.  France,  Feuille  166  (Ciermont  Ferraud). 


DYNAMICAL    GEOLOGY  447 

the  one  end  we  find  basalts  or  diabases  and  picrites,  fol- 
lowed sometimes  by  copious  outpourings  of  andesites,  while 
at  the  other  end  come  intrusions  of  felsites  and  quartz-por- 
phyries. Again,  among  the  Tertiary  lavas,  the  basalts  of 
the  great  plateaus  are  pierced  by  bosses  and  dikes  of  grano- 
phyre  and  allied  acid  rocks.  In  these  various  examples  the 
facts  point  to  some  gradual  change  in  the  composition  of  the 
subterranean  magma  during  the  lapse  of  a  single  volcanic 
period — a  change  in  which  there  was  a  separation  of  basic 
constituents  and  the  discharge  of  more  basic  lavas,  leaving 
a  more  acid  residuum  to  be  erupted  toward  the  end  of  the 
activity.168 

§5.  Causes  of  Volcanic  Action 

The  modus  operandi  whereby  the  internal  heat  of  the 
globe  manifests  itself  in  volcanic  action  is  a  problem  to 
which  as  yet  no  satisfactory  solution  has  bee_n  found.  Were 
this  action  merely  an  expression  of  the  intensity  of  the  heat, 
we  might  expect  it  to  have  manifested  itself  in  a  far  more 
powerful  manner  in  former  periods,  and  to  exhibit  a  regular- 
ity and  continuity  commensurate  with  the  exceedingly  slow 
diminution  of  the  earth's  temperature.  But  there  is  no  geo- 
logical evidence  in  favor  of  greater  volcanic  intensity  in  an- 
cient than  in  more  recent  periods;  on  the  contrary,  it  may 
be  doubted  whether  any  of  the  Palaeozoic  volcanoes  equalled 
in  magnitude  those  of  Tertiary  and  perhaps  even  post-Ter- 
tiary times.  On  the  other  hand,  no  feature  of  volcanic  ac- 
tion is  more  conspicuous  than  its  spasmodic  fitfulness.169 

As  physical  considerations  negative  the  idea  of  a  com- 

158  Quart.  Journ.  Geol.  Soc.  vol.  xlviii.  (1892),  p.  177. 

159  "Consult  Dana,    "Characteristics  of  Volcanoes,"  p.   15  et  seq.     Button, 
U.  S.   Greol.   Rep.   1882-83,   p.   183   et  seq.      Prestwicb,    Proc.   Roy.  Soc.  xli. 
(1886),  p.  117.     Lowl,  Jahrb.  Geol.  Reichsanst.   1886,  p.  315. 


448  TEXT-BOOK    OF   GEOLOGY 

paratively  thin  crust,  surmounting  a  molten  interior  whence 
volcanic  energy  might  be  derived  (ante,  p.  100),  geologists 
have  found  themselves  involved  in  great  perplexity  to  ex- 
plain volcanic  phenomena,  for  the  production  of  which  a 
source  of  no  great  depth  would  seem  to  be  necessary.  Some 
have  supposed  the  existence  of  pools  or  lakes  of  liquid  lava 
lying  beneath  the  crust,  and  at  an  inconsiderable  depth  from 
the  surface.  Others  have  appealed  to  the  influence  of  the 
contraction  of  the  earth's  mass,  assuming  the  contraction  to 
be  now  greater  in  the  outer  than  in  the  inner  portions,  and 
that  the  effect  of  this  external  contraction  must  be  to  squeeze 
out  some  of  the  internal  molten  matter  through  weak  parts 
of  the  crust. '" 

That  volcanic  action  is  one  of  the  results  of  terrestrial 
contraction  can  hardly  be  doubted,  though  we  are  still  with- 
out satisfactory  data  as  to  the  connection  between  the  cause 
and  the  effect.  It  will  be  observed  that  volcanoes  occur 
chiefly  in  lines  along  the  crests  of  terrestrial  ridges.  There 
is  probably,  therefore,  a  connection  between  the  elevation  of 
these  ridges  and  the  extravasation  of  molten  rock  at  the  sur- 
face. The  formation  of  continents  and  mountain-chains  has 
already  been  referred  to  as  probably  consequent  on  the  sub- 
sidence and  readjustment  of  the  cool  outer  shell  of  the  planet 
upon  the  hotter  and  more  rapidly  contracting  nucleus. 
Every  such  movement,  by  relieving  pressure  on  regions 
below  the  axis  of  elevation,  will  tend  to  bring  up  molten 
rock  nearer  the  surface,  and  thus  to  promote  the  formation 
and  continued  activity  of  volcanoes. 


180  Cordier,  for  example,  calculated  that  a  contraction  of  only  a  single  milli- 
metre (about  l-26th  of  an  inch)  would  suffice  to  force  out  to  the  surface  lav* 
enough  for  500  eruptions,  allowing  1  cubic  kilometre  (al-out  1300  million  cubic 
yards)  for  each  eruption.  Prof.  Prestwich  invokes  a  alight  contraction  of  the 
crust  as  the  initial  cause  of  volcanic  action.  Brit.  Assoc.  1881,  Sects,  p.  610. 


DYNAMICAL    GEOLOGY  449 

The  fissure-eruptions,  wherein  lava  has  risen  in  innu- 
merable rents  in  the  ground  across  the  whole  breadth  of  a 
country,  and  has  been  poured  out  at  the  surface  over  areas 
of  many  thousand  square  miles,  flooding  them  sometimes  to 
a  depth  of  several  thousand  feet,  undoubtedly  prove  that 
molten  rock  existed  at  some  depth  over  a  large  extent  of 
territory,  and  that,  by  some  means  still  unknown,  it  was 
forced  out  to  the  surface  (ante,  p.  434).  In  investigating 
this  subject,  it  would  be  important  to  discover  whether  any 
evidence  of  great  terrestrial  crumpling  or  other  movement 
of  the  crust  can  be  ascertained  to  have  taken  place  about  the 
same  geological  period  as  a  stupendous  outpouring  of  lava — 
whether,  for  example,  the  great  lava-fields  of  Idaho  may 
have  had  any  connection  with  contemporaneous  flexure  of 
the  North  American  mountain-system,  or  whether  the  basalt- 
plateaus  of  Antrim,  Scotland,  Faroe,  and  Iceland  may  pos- 
sibly have  been  in  their  origin  sympathetic  with  the  post- 
Eocene  upheaval  of  the  Alps  or  other  Tertiary  movements 
in  Europe.  The  most  striking  instance  of  an  apparent  con- 
nection between  such  terrestrial  disturbances  and  volcanic 
phenomena  is  that  supplied  by  the  great  semicircle  of  erup- 
tions that  sweeps  from  Central  France  by  the  Eifel,  Hoch- 
gau,  and  Bohemia  into  Hungary,  and  which  has  been  re- 
ferred to  the  dislocations  consequent  on  the  upheaval  of 
the  Alps.161 

In  the  ordinary  phase  of  volcanic  action,  marked  by  the 
copious  evolution  of  steam  and  the  abundant  production 
of  dust,  slags,  and  cinders,  from  one  or  more  local  vents, 
the  main  proximate  cause  of  volcanic  excitement  is  obvi- 


161  Sueas,  "Antlitz  der  Erde,"  i.  p.  358,  pi.  iii. ;  Julion.  Annuaire  du  Club 
Alpin,  1879-80,  p.  446;  Michel-Levy,  Bull.  Soc.  Geol.  France,  xviii.  (1890), 
pp.  690,  841. 


450  TEXT-BOOK    OF   GEOLOGY 

ously  the  expansive  force  exerted  by  vapors  dissolved  in 
the  molten  magrna  from  which  lavas  proceed.  Whether 
and  to  what  extent  these  vapors  are  parts  of  the  aboriginal 
constitution  of  the  earth's  interior,  or  are  derived  by  de- 
scent from  the  surface,  is  still  an  unsolved  problem.  The 
abundant  occlusion  of  hydrogen  in  meteorites,  and  the 
capacity  of  many  terrestrial  substances,  notably  melted 
metals,  to  absorb  large  quantities  of  gases  and  vapors 
without  chemical  combination,  and  to  emit  them  on  cool- 
ing with  eruptive  phenomena,  not  unlike  those  of  volca- 
noes, have  led  some  observers  to  conclude  that  the  gaseous 
ejections  at  volcanic  vents  are  portions  of  the  original  con- 
stitution of  the  magma  of  the  globe,  and  that  to  their  escape 
the  activity  of  volcanic  vents  is  due.  Prof.  Tschermak  in 
particular  has  advocated  this  opinion,  and  it  is  meeting  with 
increasing  acceptance."2 

On  the  other  hand,  since  so  large  a  proportion  of  the 
vapor  of  active  volcanoes  consists  of  steam,  many  geolo- 
gists have  urged  that  this  steam  has  in  great  measure  been 
supplied  by  the  descent  of  water  from  above  ground.  The 
floor  of  the  sea  and  the  beds  of  rivers  and  lakes  are  all 
leaky.  Moreover,  during  volcanic  eruptions  and  earth- 
quakes, fissures  no  doubt  open  under  the  sea,  as  they  do 
on  land,  and  allow  the  oceanic  water  to  find  access  to  the 
interior.1"  Again,  rain  sinking  beneath  the  surface  of 
the  land,  percolates  down  cracks  and  joints,  and  infil- 

169  He  has  suggested  that  if  190  cubic  kilometres,  of  the  constitution  of  cast- 
iron,  be  supposed  co  solidify  annually,  and  to  give  off  50  times  its  volume  of 
gases,  it  would  suffice  to  maintain  20,000  active  volcanoes.  Sitz.  Akad.  Wissen. 
Wien,  lixv.  (1877),  p.  151.  Reyer  ("Beitrag  zur  Physik  der  Eruptionen," 
Vienna,  1877)  adyocates  the  same  view. 

us  prof.  Moseley  mentions  that  during  a  submarine  eruption  off  Hawaii  in 
1877  "a  fissure  opened  on  the  coast  of  that  island,  from  a  few  inches  to  three 
feet  broad,  and  in  some  places  the  water  was  seen  pouring  down  the  opening 
into  the  abyss  below."  "Notes  by  a  Naturalist  on  the  'Challenger,'  "  p.  503. 


DYNAMICAL    GEOLOGY  451 

trates  through  the  very  pores  of  the  rocks.  The  presence 
of  nitrogen  among  the  gaseous  discharges  of  volcanoes  may 
indicate  the  decomposition  of  water  containing  atmospheric 
gases.  The  abundant  sublimations  of  chlorides  are  such  as 
might  probably  result  from  the  decomposition  of  sea-water. 
To  some  extent  surface-waters  doubtless  do  reach  the  vol- 
canic magma. 

Whatever  may  be  its  source,  we  cannot  doubt  that  to 
the  enormous  expansive  force  of  superheated  water  (or  of 
its  component  gases,  dissociated  by  the  high  temperature), 
in  the  molten  magma  at  the  roots  of  volcanoes,  the  explo- 
sions of  a  crater  and  the  subsequent  rise  of  a  lava-column 
are  mainly  due.  The  water  or  gas  dissolved  in  the  lava  is 
retained  there  by  the  enormous  overlying  pressure  of  the 
lava-column,  but  when  the  molten  material  is  brought  up 
to  the  surface  the  pressure  is  relieved  and  the  water  vapor- 
izes and  escapes.  Where  the  relief  is  rapid  the  effect  may 
be  to  froth  up  the  lava  into  a  pasty  mass  of  pumice,  while 
where  it  is  sudden  and  extreme  the  escape  of  the  water- 
vapor  may  be  by  an  explosive  discharge. 

It  has  been  supposed  that,  somewhat  like  the  reservoirs 
in  which  hot  water  and  steam  accumulate  under  geysers, 
reservoirs  of  molten  rock  receive  a  constant  influx  of  water 
from  the  surface,  which  cannot  escape  by  other  channels, 
but  is  absorbed  by  the  internal  magma  at  an  enormously 
high  temperature  and  under  vast  pressure.  In  the  course 
of  time,  the  materials  filling  up  the  chimney  are  unable  to 
withstand  the  upward  expansion  of  this  imprisoned  vapor 
or  water-substance,  so  that,  after  some  premonitory  rum- 
blings, the  whole  opposing  mass  is  blown  out,  and  the 
vapor  escapes  in  the  well-known  masses  of  cloud.  Mean- 
while, the  removal  of  the  overlying  column  relieves  the 


•±52  TEXT-BOOK    OF   GEOLOGY 

pressure  on  the  lava  underneath,  saturated  with  vapors  or 
superheated  water.  This  lava  therefore  begins  to  rise  in 
the  funnel  until  it  forces  its  way  through  some  weak  part 
of  the  cone,  or  pours  over  the  top  of  the  crater.  After  a 
time,  the  vapor  being  expended,  the  energy  of  the  volcano 
ceases,  and  there  comes  a  variable  period  of  repose,  until 
a  renewal  of  the  same  phenomena  brings  on  another  erup- 
tion. By  such  successive  paroxysms,  the  forms  of  the  in- 
ternal reservoirs  and  tunnels  may  be  changed;  new  spaces 
for  the  accumulation  of  superheated  water  being  opened, 
whence  in  time  fresh  volcanic  vents  issue,  while  the  old 
ones  gradually  die  out. 

An  obvious  objection  to  this  explanation  is  the  difficulty 
of  conceiving  that  water  should  descend  at  all  against  the 
expansive  force  within.  .But  Daubree's  experiments  have 
shown  that,  owing  to  capillarity,  water  may  permeate  rocks 
against  a  high  counter-pressure  of  steam  on  the  further  side, 
and  that  so  long  as  the  water  is  supplied,  whether  by  minute 
fissures  or  through  pores  of  the  rocks,  it  may,  under  pres- 
sure of  its  own  superincumbent  column,  make  its  way  into 
highly  heated  regions.164  Experience  in  deep  mines,  how- 
ever, rather  goes  to  show  that  the  permeation  of  water 
through  the  pores  of  rocks  gets  feebler  as  we  descend. 

Reference  may  be  made  here  to  a  theory  of  volcanic 
action  in  which  the  influence  of  terrestrial  contraction  as 
the  grand  source  of  volcanic  energy  was  insisted  upon  by  the 
late  Mr.  Mallet.185  He  maintained  that  all  the  present  mani- 

164  Daubree,  "G-eologie  Experimental, "  p.  274  (criticised  adversely  by  Fisher, 
"Physics  of  Earth's  Crust,"  2d  ed.  p.  144).  Tschermak,  cited  on  page  450. 
Reyer,  "Beitrag  zur  Physik  der  Eruptionen,"  §  1. 

1M  Phil.  Trans.  1873.  See  also  Daubree's  experimental  determination  of  the 
quantity  of  heat  evolved  by  the  internal  crushing  of  rocks.  "Geologie  Experi- 
mentale,"  p.  448.  For  an  adverse  criticism  of  Mallet's  view,  see  Fisher,  op.  cit. 
chap.  xxii. 


DYNAMICAL    GEOLOGY  453 

festations  of  hypogene  action  are  due  directly  to  the  more 
rapid  contraction  of  the  hotter  internal  mass  of  the  earth 
and  the  consequent  crushing  in  of  the  outer  cooler  shell. 
He  pointed  to  the  admitted  difficulties  in  the  way  of  con- 
necting volcanic  phenomena  with  the  existence  of  internal 
lakes  of  liquid  matter,  or  of  a  central  ocean  of  molten  rock. 
Observations  made  by  him,  on  the  effects  of  the  earthquake 
shocks  accompanying  the  volcanic  eruptions  of  Vesuvius 
and  of  Etna,  showed  that  the  focus  of  disturbance  could 
not  be  more  than  a  few  miles  deep;  that,  in  relation  to  the 
general  mass  of  the  globe,  it  was  quite  superficial,  and  could 
not  possibly  have  lain  under  a  crust  of  800  miles  or  upward 
in  thickness.  The  occurrence  of  volcanoes  in  lines,  and 
especially  along  some  of  the  great  mountain-chains  of  the 
planet,  was  likewise  dwelt  upon  by  him  as  a  fact  not  satis- 
factorily explicable  on  any  previous  hypothesis  of  volcanic 
energy.  But  he  contended  that  all  these  difficulties  disap- 
pear when  once  the  simple  idea  of  cooling  and  contraction 
is  adequately  realized.  "The  secular  cooling  of  the  globe," 
he  remarks,  "is  always  going  on,  though  in  a  very  slowly 
descending  ratio.  Contraction  is  therefore  constantly  pro- 
viding a  store  of  energy  to  be  expended  in  crushing  parts 
of  the  crust,  and  through  that  providing  for  the  volcanic 
heat.  But  the  crushing  itself  does  not  take  place  with  uni- 
formity; it  necessarily  acts  per  saltum  after  accumulated 
pressure  has  reached  the  necessary  amount  at  a  given 
point,  where  some  of  the  pressed  mass,  unequally  pressed 
as  we  must  assume  it,  gives  way,  and  is  succeeded  perhaps 
by  a  time  of  repose,  or  by  the  transfer  of  the  crushing 
action  elsewhere  to  some  weaker  point.  Hence,  though 
the  magazine  of  volcanic  energy  is  being  constantly  and 
steadily  replenished  by  secular  cooling,  the  effects  are 


454  TEXT-BOOK   OF   GEOLOGY 

intermittent."  He  offered  an  experimental  proof  of  the 
sufficiency  of  the  store  of  heat  produced  by  this  internal 
crushing  to  cause  all  the  phenomena  of  existing  volca- 
noes.18" The  slight  comparative  depth  of  the  volcanic  foci, 
their  linear  arrangement,  and  their  occurrence  along  lines 
of  dominant  elevation  become,  he  contended,  intelligible 
under  this  hypothesis.  For  since  the  crushing  in  of  the 
crust  may  occur  at  any  depth,  the  volcanic  sources  may 
vary  in  depth  indefinitely;  and  as  the  crushing  will  take 
place  chiefly  along  lines  of  weakness  in  the  crust,  it  is 
precisely  in  such  lines  that  crumpled  mountain-ridges  and 
volcanic  funnels  should  appear.  Moreover,  by  this  expla- 
nation its  author  sought  to  harmonize  the  discordant  obser- 
vations regarding  variations  in  the  rate  of  increase  of  tem- 
perature downward  within  the  earth,  which  have  already 
been  cited  and  referred  to  unequal  conductivity  in  the  crust 
(p.  97).  He  pointed  out  that  in  some  parts  of  the  crust  the 
crushing  must  be  much  greater  than  in  other  parts;  and 
since  the  heat  "is  directly  proportionate  to  the  local  tan- 
gential pressure  which  produces  the  crushing  and  the  re- 
sistance thereto,"  it  may  vary  indefinitely  up  to  actual 
fusion.  So  long  as  the  crushed  rock  remains  out  of  reach 
of  a  sufficient  access  of  subterranean  water,  there  would,  of 
course,  be  no  disturbance.  But  if,  through  the  weaker 
parts,  water  enough  should  descend  and  be  absorbed  by 
the  intensely  hot  crushed  mass,  it  would  be  raised  to  a  very 
high  temperature,  and,  on  sufficient  diminution  of  pressure, 


166  The  elaborate  and  careful  experimental  researches  of  this  observer  will 
reward  attentive  perusal.  Mallet  estimates  from  experiment  the  amount  of  heat 
given  out  by  the  crushing  of  different  rocks  (syenite,  granite,  sandstone,  slate, 
limestone),  and  concludes  that  a  cubic  mile  of  the  crust  taken  at  the  mean  den- 
sity would,  if  crushed  into  powder,  give  out  heat  enough  to  melt  nearly  3$  cubic 
miles  of  similar  rock,  assuming  the  melting-point  to  be  2000°  Fahr. 


DYNAMICAL    GEOLOGY  455 

would  flash  into  steam  and  produce  the  commotion  of  a  vol- 
canic eruption. 

This  ingenious  theory  requires  the  operation  of  sudden 
and  violent  movements,  or  at  least  that  the  heat  generated 
by  the  crushing  should  be  more  than  can  be  immediately 
conducted  away  through  the  crust.  Were  the  crushing  slow 
and  equable,  the  heat  developed  by  it  might  be  so  tran- 
quilly dissipated  that  the  temperature  of  the  crust  would 
not  be  sensibly  affected  in  the  process,  or  not  to  such  an 
extent  as  to  cause  any  appreciable  molecular  rearrange- 
ment of  the  particles  of  the  rocks.  But  an  amount  of 
internal  crushing  insufficient  to  generate  volcanic  action 
may  have  been  accompanied  by  such  an  elevation  of  tem- 
perature as  to  induce  important  changes  in  the  structure 
of  rocks,  such  as  are  embraced  under  the  term  "meta- 
morphic." 

There  is,  indeed,  strong  evidence  that,  among  the  conse- 
quences arising  from  the  secular  contraction  of  the  globe, 
masses  of  sedimentary  strata,  many  thousands  of  feet  in 
thickness,  have  been  crumpled  and  crushed,  and  that  the 
crumpling  has  often  been  accompanied  by  such  an  amount 
of  heat  and  evolution  of  chemical  activity  as  to  produce 
an  interchange  and  rearrangement  of  the  elements  of  the 
rocks — this  change  sometimes  advancing  perhaps  to  the 
point  of  actual  fusion.  (See  postea,  p.  506,  and  Book  IV. 
Part  VIII.)  There  is  reason  to  believe  that  some  at  least  of 
these  periods  of  intense  terrestrial  disturbance  have  been 
followed  by  periods  of  prolonged  volcanic  action  in  the 
disturbed  areas.  Mr.  Mallet's  theory  is  thus,  to  some  ex- 
tent, supported  by  independent  geological  testimony.  The 
existence,  however,  of  large  reservoirs  of  fused  rock,  at  a 
comparatively  small  depth  beneath  the  surface,  may  be  con- 


456  TEXT-BOOK    OF   GEOLOGY 

ceived  as  probable,  apart  from  the  effects  of  crushing.  The 
connection  of  volcanoes  with  lines  of  elevation,  and  conse- 
quent weakness  in  the  earth's  crust,  is  what  might  have 
been  anticipated  on  the  view  that  the  nucleus,  though 
practically  solid,  is  at  such  a  temperature  and  pressure 
that  any  diminution  of  the  pressure,  by  corrugation  of  the 
crust  or  otherwise,  will  cause  the  subjacent  portion  of 
the  nucleus  to  melt.  Along  lines  of  elevation  the  pressure 
is  relievecf,  and  consequent  melting  may  take  place.  On 
these  lines  of  weakness  and  fracture,  therefore,  the  condi- 
tions for  volcanic  excitement  may  be  conceived  to  be  best 
developed,  whether  arising  from  the  explosive  energy  of 
water  dissolved  in  the  magma  or  from  water  descending 
to  the  intensely  heated  materials  underneath  the  crust. 
The  periodicity  of  eruptions  may  thus  depend  upon  the 
length  of  time  required  for  the  storing  up  of  sufficient 
steam,  and  on  the  amount  of  resistance  in  the  crust  to  be 
overcome.  In  some  volcanoes,  the  intervals  of  activity, 
like  those  of  many  geysers,  return  with  considerable  regu- 
larity. In  other  cases,  the  shattering  of  the  crust,  or  the 
upwelling  of  vast  masses  of  lava,  or  the  closing  of  subter- 
ranean passages  for  the  descending  water,  or  other  causes 
may  vary  the  conditions  so  much,  from  time  to  time,  that 
the  eruptions  follow  each  other  at  very  unequal  periods, 
and  with  very  discrepant  energy.  Each  great  outburst  ex- 
hausts for  a  while  the  vigor  of  the  volcano,  and  an  interval 
is  needed  for  the  renewed  accumulation  of  vapor. 

But  besides  the  mechanism  by  which  volcanic  eruptions 
are  produced,  further  problems  are  presented  by  the  varie- 
ties of  materials  ejected,  by  the  differences  which  these  ex- 
hibit at  neighboring  vents,  even  sometimes  in  successive 
eruptions  from  the  same  vent,  by  the  alternation  or  recur- 


DYNAMICAL    GEOLOGY  457 

rence  of  lavas  from  basic  to  acid  in  the  continuance  of  a  sin- 
gle volcanic  period,  and  by  the  repetition  of  a  similar  cycle 
in  successive  periods.  Observations  are  yet  needed  from  a 
larger  number  of  ancient  volcanic  districts  and  in  greater  de- 
tail, before  these  problems  can  be  satisfactorily  discussed 
and  solved.  It  is  obvious  that  in  such  a  great  series  of 
eruptions  as  that  of  Central  France,  where  over  a  compara- 
tively limited  area  an  alternation  of  basic  and  acid  lavas  has 
been  many  times  repeated,  the  subterranean  magma  must 
have  undergone  a  succession  of  changes  in  composition. 
Perhaps  a  definite  cycle  of  such  alternations  may  be  made 
out.  The  sequence  from  basic  to  acid  protrusions,  observ- 
able among  the  British  Palaeozoic  volcanic  rocks,  is  sugges- 
tive of  a  separation  of  the  more  basic  constituents  of  the 
magma  with  consequent  increasing  acidity  of  the  residue. 
The  earliest  lavas  mark  the  more  basic  condition  of  the 
magma,  while  the  latest  felsite  and  quartz-porphyry  intru- 
sions show  its  impoverishment  in  bases  at  the  close  of  a  vol- 
canic period.  During  the  interval  before  the  next  period 
the  magma  had  in  some  way  been  renewed,  for  when  erup- 
tions began  anew  they  were  once  more  basic.  But  by  the 
close  of  the  volcanic  activity  the  magma  had  again  lost  a 
large  proportion  of  its  basic  constituents  and  had  become 
decidedly  acid. 

Reference  has  already  (p.  114)  been  made  to  the  specula- 
tion of  Durocher  as  to  the  existence  within  the  crust  of  an 
upper  siliceous  layer  with  a  mean  of  71  per  cent  of  silica, 
and  a  lower  basic  layer  with  about  51  per  cent  of  silica. 
Bunsen  also  came  to  the  conclusion  that  volcanic  rocks  are 
mixtures  of  two  original  normal  magmac  —the  normal  tra- 
chytic  (with  a  mean  of  76*67  silica),  and  the  normal  pyroxenic 
(with  a  mean  of  47-48  silica).  The  varying  proportions 

GEOLOGY— Yol.  XXIX— 20 


458  TEXT-BOOK    OF   GEOLOGY 

in  which  these  two  original  magmas  have  been  combined 
are,  in  Bunsen's  view,  the  cause  of  the  differences  of  vol- 
canic rocks.  We  may  conceive  these  two  layers  to  be 
superposed  upon  each  other,  according  to  relative  densities, 
and  the  composition  of  the  last  material  erupted  at  the  sur- 
face to  depend  upon  the  depth  from  which  it  has  been  de- 
rived.1" The  earliest  explosions  may  be  supposed  to  have 
taken  place  usually  from  the  upper  lighter  and  more  sili- 
ceous layer,  and  the  lavas  ejected  would  consequently  be  in 
general  acid,  while  later  eruptions,  reaching  down  to  deeper 
and  heavier  zones  of  the  magma,  brought  up  such  basic 
lavas  as  basalt.  Certainly  the  general  similarity  of  the  vol- 
canic rocks  all  over  the  globe  would  appear  to  prove  that 
there  must  be  considerable  uniformity  of  composition  in  the 
zones  of  intensely  hot  material  from  which  volcanic  rocks 
are  derived.1" 

Many  difficulties,  however,  remain  yet  to  be  explained 
before  our  knowledge  of  volcanic  action  can  be  regarded  as 
more  than  rudimentary.  In  Book  IV.  Part  VII.  a  descrip- 
tion is  given  of  the  part  volcanic  rocks  have  played  in  build- 
ing up  what  we  see  of  the  earth's  crust,  and  the  student  will 
there  find  other  illustrations  of  facts  and  deductions  which 
have  been  given  in  the  previous  pages. 


141  See  R.  Bunsen,  Fogg.  Ann.  Ixiii.  (1851),  p.  204;  Sarlorius  von  Walters- 
hausen,  "Sicilien  und  Island,"  p.  416;  Reyer,  "Beitrag  zur  Physik  der  Erup- 
tionen,"  iii.  Scrope  had  long  before  suggested  a  classification  of  volcanic  rock* 
into  Trachyte,  Graystone,  and  Basalt,  Jouru.  Science,  xxi. 

168  In  the  memoir  by  Captain  Button,  cited  in  a  previous  note,  the  hypothe- 
sis is  maintained  that  the  order  of  appearance  of  the  lavas  is  determined  by  their 
relative  density  and  fusibility,  the  most  basic  and  heaviest,  though  most  easily 
fused,  requiring  the  highest  temperature  to  diminish  their  density  to  such  an 
extent  as  to  permit  them  to  be  erupted. 


DYNAMICAL    GEOLOGY  469 

Section  ii.    Earthquakes169 

By  the  more  delicate  methods  of  observation  which  have 
been  invented  in  recent  years,  it  has  been  ascertained  that 
the  ground  beneath  our  feet  is  apparently  everywhere  sub- 
ject to  continual  slight  tremors  and  to  minute  pulsations  of 
longer  duration.  The  old  expression  "terra  firma"  is  not 
only  not  strictly  true,  but  in  the  light  of  modern  research 
seems  singularly  inappropriate.  Rapid  changes  of  tempera- 
ture and  atmospheric  pressure,  the  fall  of  a  shower  of  rain, 
the  patter  of  birds'  feet,  and  still  more  the  tread  of  larger 
animals,  produce  tremors  of  the  ground  which,  though  ex- 
ceedingly minute,  are  capable  of  being  made  clearly  audible 
by  means  of  the  microphone  and  visible  by  means  of  the 
galvanometer.  Some  tremors  of  varying  intensity  and  ap- 
parently of  irregular  occurrence,  may  be  due  to  minute 
movements  or  displacements  in  the  crust  of  the  earth.  Less 

169  On  the  phenomena  of  earthquakes  consult  Mallet,  Brit.  Assoc.  1847,  part 
ii.  p.  30;  1850,  p.  1;  1851,  p.  272;  1852,  p.  1;  1858,  p.  1;  1861,  p.  201;  "The 
Great  Neapolitan  Earthquake  of  1857,"  2  vols.,  1862;  D.  Milne,  Edin.  New 
Phil.  Journ.  xxxi.— xxxvt ;  A.  Perrey,  Mem.  Couronn.  Bruxelles,  xviii.  (1844), 
Comptes  rendus,  lii.  p.  146;  Otto  Volger,  "Untersuchungen  fiber  die  Phano- 
mene  der  Erdbeben  in  der  Schweiz,"  Gotha,  1857-58;  Z.  Deutsch.  Geol.  Ges. 
xiii.  p.  667;  R.  Falb,  "Grundziige  einer  Theorie  der  Erdbeben  und  Vulkanens- 
ausbriiche,"  Graz,  1871;  "Gedanken  und  Studien  iiber  den  Vulkanismus,  etc.," 
1874;  Pfaff,  "Allgemeine  Geologie  als  exacte  Wissenschaft,"  Leipzig,  1873, 
p.  224.  Records  of  observed  earthquakes  will  be  found  in  the  memoirs  of 
Mallet  and  Perrey;  also  in  papers  by  Fuchs  in  Neues  Jahrb.  1865-1871,  and 
in  Tschermak's  Mineralog.  Mittheilungen,-1873  and  subsequent  years.  See  also 
Schmidt,  "Studien  iiber  Erdbeben,"  2d  edit.  1879;  "Studien  iiber  Vulkane  und 
Erdbeben,"  1881 ;  Dieffenbach,  Neues  Jahrb.  1872,  p.  155;  M.  S.  di  Rossi,  "La 
Meteorologia  Endogena,"  2  vols.  1879  and  1882;  M.  Gatta,  "L'ltalia,  su  vul- 
cani  e  terremoti,"  1882;  J.  Milne,  "Earthquakes  and  other  Earth -movements, " 
1886,  and  his  beautifully  illustrated  volume  on  the  Japan  Earthquake  of  Octo- 
ber, 1891.  G.  Mercalli,  in  his  "Vulcani  e  Fenomeni  Vulcanici  in  Italia"  (1883), 
gives  an  account  of  the  Italian  earthquakes  from  1450  B.C.  to  A.D.  1881;  he 
separately  describes  the  great  Ischian  earthquake  of  1883;  "L'Isola  d'Ischia," 
Milan,  1884.  Much  interesting  information  will  be  found  in  the  Bulletino  del 
Vulcanismo  Italiano,  which  began  to  be  published  in  1874;  also  in  the  Transac- 
tions of  the  Seismological  Society  of  Japan — a  society  instituted  in  the  year  1880 
for  the  invescigation  of  earthquake  phenomena,  especially  in  Japan,  where  they 
are  of  frequent  occurrence.  Other  papers  are  quoted  in  the  following  pages. 


460  TEXT-BOOK   OF   GEOLOGY 

easily  traceable  are  the  slow  pulsations  of  the  crust,  which 
in  many  cases  are  periodic,  and  may  depend  on  such  causes 
as  the  diurnal  oscillation  of  the  thermal  or  barometric  con- 
ditions of  the  atmosphere,  the  rise  and  fall  of  the  tides,  etc. 
So  numerous  and  well  marked  are  these  tremors  and  pulsa- 
tions, that  the  delicate  observations  which  were  set  on  foot 
to  determine  the  lunar  disturbance  of  gravity  had  to  be 
abandoned,  for  it  was  found  that  the  minute  movements 
sought  for  were  wholly  eclipsed  by  these  earth  tremors.1" 

The  term  Earthquake  denotes  any  natural  subterranean 
concussion,  varying  from  such  slight  tremors  as  to  be  hardly 
perceptible  up  to  severe  shocks,  by  which  houses  are  lev- 
elled, rocks  dislocated,  landslips  precipitated,  and  many 
human  lives  destroyed.  The  phenomena  are  analogous  to 
the  shock  communicated  to  the  ground  by  explosions  of 
mines  or  powder-works.  They  may  be  most  intelligibly 
considered  as  wave-like  undulations  propagated  through 
the  solid  crust  of  the  earth.  In  Mr.  Mallet's  language,  an 
earthquake  may  be  denned  as  "the  transit  of  a  wave  of 
elastic  compression,  or  of  a  succession  of  these,  in  parallel 
or  intersecting  lines  through  the  solid  substance  and  sur- 
face of  the  disturbed  country."  Mr.  Milne  has  since  re- 
marked that  the  disturbance  may  also  be  due  to  the  transit 
of  waves  of  elastic  distortion.  The  passage  of  the  wave  of 
shock  constitutes  the  real  earthquake. 

Besides  the  wave  of  shock  transmitted  through  the  solid 

110  A.  d'Abbadie,  "Etudes  sur  la  verticale,"  1872.  Plantamour,  Comptes 
rend.  June,  1878,  February,  1881;  Archives  Sciences  Phys.  Nat.  Geneva,  ii.  p. 
641;  v.  p.  97;  vii.  p.  601;  viii.  p.  551;  x.  p.  616;  xii.  (1884),  p.  388.  G.  H. 
Darwin,  Brit.  Assoc.  1882,  p.  95.  In  this  paper  Prof.  Darwin  discusses  the 
amount  of  disturbance  of  the  vertical  near  the  coasts  of  continents,  caused  by 
the  rise  and  fall  of  the  tide.  J.  Milne,  Trans.  Seismological  Soc.  Japan,  vi. 
(1883),  p.  1;  Geol.  Mag.  1882,  p.  482;  Nature,  xxvi.  p.  125.  The  numerous 
observations  made  by  Rossi  in  Italy  are  summarized  by  G.  Mercalli  in  his  work 
cited  above,  p.  332. 


DYNAMICAL    GEOLOGY  461 

crust,  waves  are  also  propagated  through  the  air,  and,  where 
the  site  of  the  impulse  is  not  too  remote,  through  the  ocean. 
Earthquakes  originating  under  the  sea  are  numerous  and 
specially  destructive  in  their  effects.  They  illustrate  well 
the  three  kinds  of  waves  associated  with  the  progress  of  an 
earthquake.  These  are,  1st,  The  true  earth-wave  through 
the  earth's  crust;  2d,  A  wave  propagated  through  the  air,  to 
which  the  characteristic  sounds  of  rolling  wagons,  distant 
thunder,  bellowing  oxen,  etc.,  are  due;  3d,  Two  sea- waves, 
one  of  which  travels  on  the  back  of  the  earth- wave  and 
reaches  the  land  with  it,  producing  no  sensible  effect  on 
shore;  the  other  an  enormous  low  swell,  caused  by  the  first 
sudden  blow  of  the  earth-wave,  but  travelling  at  a  much 
slower  rate,  and  reaching  land  often  several  hours  after  the 
earthquake  has  arrived. 

Amplitude  of  earth-movements.— The  popular  conception 
of  the  extent  to  which  the  ground  moves  to  and  fro  or  up 
and  down  during  an  earthquake  is  a  great  exaggeration  of 
the  truth.  As  the  result  of  very  careful  measurement  with 
delicate  instruments,  there  appears  to  be  reason  to  believe 
that  the  horizontal  motion  at  the  time  of  a  small  earthquake 
is  usually  only  the  fraction  of  a  millimetre,  and  seldom  ex- 
ceeds three  or  four  millimetres.  When  the  motion  rises  to 
five  or  six  millimetres  brick  and  stone  chimneys  are  shat- 
tered. Yet  even  with  such  an  intensity  of  shock  a  person 
walking  in  an  open  place  might  be  quite  unconscious  of  any 
perceptible  movement  of  the  ground.  The  vertical  motion 
also  appears  to  be  exceedingly  small.1" 

Velocity* — Experiments  have  been  made  to  determine  the 

111  Milne  "Earthquakes,"  pp.  75,  76.  An  ingenious  model  in  wire  has  been 
made  by  Prof.  Sekiya  to  illustrate  the  highly  complex  path  pursued  by  a  parti- 
cle on  the  surface  of  the  ground  during  an  earthquake  at  Tokio,  Japan,  on  15th 
January,  1887. 


462  TEXT-BOOK    OF   GEOLOGY 

velocity  of  the  earth-wave,  and  its  variation  with  the  nature 
of  the  material  through  which  it  is  propagated.  Mr.  Mallet 
found  that  the  shock  produced  by  the  explosion  of  gunpow- 
der travelled  at  the  rate  per  second  of  825  feet  in  sand ;  1088 
feet  in  schists,  slates,  and  quartzites;  1306  feet  in  friable 
granite;  and  1664  feet  in  solid  granite.  General  Abbot,  by 
observing  the  effects  of  the  explosion  of  dynamite  and  gun- 
powder, found  the  velocity  of  transmission  of  the  shock  to 
vary  from  1240  to  8800  feet  per  second,  and  to  be  greatest 
where  the  shock  is  most  violent.172  Observations  of  the 
time  at  which  an  earthquake  has  successively  visited  the 
different  places  on  its  track  have  shown  similar  variations  in 
the  rate  of  movement.  Thus  in  the  Calabrian  earthquake 
of  1857,  the  wave  of  shock  varied  from  658  to  989  feet  per 
second,  the  mean  rate  being  789  feet.  The  earthquake  at 
Viege  in  1855  was  estimated  to  have  travelled  northward 
toward  Strasburg  at  the  rate  of  2861  feet  per  second,  and 
southward  toward  Turin  at  a  rate  of  1398  feet,  or  less  than 
half  the  northern  speed.  The  earthquake  of  7th  October, 
1874,  in  northern  Italy,  travelled  at  rates  varying  from  273 
to  874  feet  per  second.  That  of  12th  March,  1873,  showed 
a  velocity  per  second  of  2734  feet  between  Eagusa  and 
Venice;  4101  feet  from  Spoleto  to  Venice;  601  feet  from 
Perugia  to  Orvieto;  1640  feet  from  Perugia  to  Ancona;  and 
1640  (or  2188)  feet  from  Perugia  to  Kome.  The  rate  of  the 
central  European  earthquake  of  1872  was  estimated  to  have 
been  2433  feet,  that  of  Herzogenrath,  June  24,  1877,- 1555 
feet,  that  of  an  earthquake  at  Travancore,  in  Southern  Hin- 


112  Amer.  Journ.  Sci.  xv.  (1878).  Prof.  J.  Milne,  experimenting  in  Japan, 
has  likewise  ascertained  that  a  close  relation  exists  between  the  initial  violence 
of  the  shock  and  the  velocity  of  propagation,  and  that  there  is  a  progressive 
diminution  in  speed  as  the  wave  of  shock  travels  outward  from  the  centre  of 
disturbance.  "Earthquakes,"  p.  65. 


DYNAMICAL    GEOLOGY  463 

dustan,  656  feet  in  a  second."8  The  most  accurate  measure- 
ments and  computations  of  the  velocity  of  earthquake  move- 
ments are  probably  those  made  by  Prof.  J.  Milne  and  his 
associates  in  Japan.  The  rates  of  movement  during  the 
Tokio  earthquake  of  25th  October,  1881,  are  estimated  to 
have  ranged  between  4000  and  9000  feet  per  second.  As  the 
result  of  prolonged  observation,  Prof.  Milne  concludes  that 
"different  earthquakes,  although  they  may  travel  across  the 
same  country,  have  very  variable  velocities,  varying  be- 
tween several  hundreds  and  several  thousands  of  feet  per 
second;  that  the  same  earthquake  travels  more  quickly 
across  districts  near  to  its  origin  than  it  does  across  dis- 
tricts which  are  far  removed;  and  that  the  greater  the  inten- 
sity of  the  shock,  the  greater  is  the  velocity."  "* 

Duration* — The  number  of  shocks  in  an  earthquake 
varies  indefinitely,  as  well  as  the  length  of  the  intervals 
between  them.  Sometimes  the  whole  earthquake  only  lasts 
a  few  seconds;  thus  the  city  of  Caracas,  with  its  fine 
churches  and  10,000  of  its  inhabitants,  was  destroyed  in 
about  half  a  minute;  Lisbon  was  overthrown  in  five  min- 
utes. But  a  succession  of  shocks  of  varying  intensity  may 
continue  for  days,  weeks,  or  months.  The  Calabrian  earth- 
quake, which  began  in  February,  1783,  was  continued  by 
repeated  shocks  for  nearly  four  years  until  the  end  of  1786. 

Modifying  influence  of  geological  structure. — In  its  pas- 


"»  K.  von  Seebach,  "Das  Mitteldeutsche  Erdbeben  von  6  Marz,  1872," 
Leipzig,  1873.  Hofer,  Sitzb.  Akad.  Wien,  Dec.  1876;  A.  von  Lasaulx,  "Das 
Erdbeben  von  Herzogenrath,  22d  Oct.,  1873,"  Bonu,  1874.  "Das  Erdbebea 
von  Herzogenrath,  24  Juni,  1877,"  Bonn,  1878.  G.  C.  Laube,  on  Earthquake 
of  31st  January,  1883,  at  Trautenau,  Jahrb.  Geol.  Reichs.  1883,  p.  331.  H. 
Credner,  on  the  Earthquakes  of  the  Erzgebirge  and  Vogtiand  from  1878  to  1884, 
Zeitsch.  fur  Naturwiss.  vol.  Ivii.  (1884).  P.  Wahner,  on  Agram  earthquake  of 
9  Nov.  1880,  Sitz.  Akad.  Wien,  Izxxviii.  (1883),  p.  15.  Di  Rossi,  "Meteorologia 
Endogena,"  i.  p.  306;  P.  Serpieri,  Institute  Lombardo,  1873. 

"4  "Earthquakes,"  p.  94. 


464  TEXT-BOOK    OF   GEOLOGY 

sage  through  the  solid  terrestrial  crust  from  the  focus  of 
origin,  the  earth-wave  must  be  liable  to  continual  deflec- 
tions and  delays,  from  the  varying  geological  structure  of 
the  rocks.  To  this  cause,  no  doubt,  must  be  in  large  meas- 
ure ascribed  the  marked  differences  in  the  rate  of  propaga- 
tion of  the  same  earthquake  in  different  directions.  The 
wave  of  disturbance,  as  it  passes  from  one  kind  of  rock  to 
another,  and  encounters  materials  of  very  different  elas- 
ticity, or  as  it  meets  with  joints,  dislocations,  and  curva- 


Fig  71.— Plan  of  Port  Royal,  Jamaica,  showing  the  effects  of  the  Earthquake 
of  1692  (B.). 

P  C,  Portions  of  the  town  built  on  limestone  and  left  standing  after  the  earthquake; 
a  a,  L,  the  boundary  of  the  town  prior  to  the  earthquake;  N  N,  Ground  gained  by 
the  drifting  of  sand  up  to  the  end  of  last  century;  I  L,  H,  Additions  from  the  same 
cause  during  the  first  quarter  of  the  present  century. 

tures  in  the  same  rock,  must  be  liable  to  manifold  changes 
alike  in  rate  and  in  direction  of  movement.  Even  at  the 
surface,  one  effect  of  differences  of  material  may  be  seen 
in  the  apparently  capricious  demolition  of  certain  quarters 
of  a  city,  while  others  are  left  comparatively  scathless. 
In  such  cases,  it  has  often  been  found  that  buildings 
erected  on  loose  inelastic  foundations,  such  as  sand  and 
clay,  are  more  liable  to  destruction  than  those  placed  upon 
solid  rock.  In  illustration  of  this  statement  the  accom- 
panying plan  (Fig.  71)  of  Port  Royal,  Jamaica,  was  given 


DYNAMICAL    GEOLOGY  465 

by  De  la  Beche175  to  show  that  the  portions  of  the  town 
which  did  not  disappear  during  the  earthquake  of  1692 
were  built  upon  solid  white  limestone,  while  the  parts 
built  on  sand  were  shaken  to  pieces.178 

It  has  been  observed  that  an  earthquake  shock  will 
pass  under  a  limited  area  without  disturbing  it,  while  the 
region  all  around  has  been  affected,  as  if  there  were  some 
superficial  stratum  protected  from  the  earth-wave.  Hum- 
boldt  cited  a  case  where  miners  were  driven  up  from  below 
ground  by  earthquake  shocks  not  perceptible  at  the  surface, 
and  on  the  other  hand,  an  instance  where  they  experienced 
no  sensation  of  an  earthquake  which  shook  the  surface  with 
considerable  violence.177  Such  facts  bring  impressively  be- 
fore the  mind  the  extent  to  which  the  course  of  the  earth- 
wave  must  be  modified  by  geological  structure.  In  some 
instances,  the  shock  extends  outward  from  a  common 
centre,  so  that  a  series  of  concentric  circles  may  be  drawn 
round  the  focus,  each  of  which « will  denote  a  certain  ap- 
proximately uniform  intensity  of  shock  ("coseismic  lines" 
of  Mallet),  this  intensity,  of  course,  diminishing  with  dis- 
tance from  the  focus.  The  Calabrian  earthquake  of  1857 
and  that  of  Central  Europe  in  1872  may  be  taken  in  illus- 
tration of  this  central  type.  In  other  cases,  however,  the 
earthquake  travels  chiefly  along  a  certain  band  or  zone 
(particularly  along  the  flanks  of  a  mountain-chain)  without 
advancing  far  from  it  laterally.  This  type  of  linear  earth- 
quake is  exemplified  by  the  frequent  shocks  which  traverse 


1  "Geological  Observer,"  p.  426. 

"'  The  opposite  effect  has  been  observed  on  the  island  of  Ischia,  the  houses 
built  on  loose  subsoil  generally  having  suffered  much  less  than  the  others.  There 
appears,  indeed,  to  be  a  considerable  conflict  of  testimony  on  this  subject.  See 
Milne,  "Earthquakes,"  p.  130. 

m  "Cosmos,"  Art.  Earthquakes. 


466  TEXT-BOOK    OF    GEOLOGY 

Chile,  Peru,  and  Ecuador,  between  the  line  of  the  Andes 
and  the  Pacific  coast.178 

Extent  of  country  affected.— The  area  shaken  by  an  earth- 
quake varies  with  the  intensity  of  the  shock,  from  a  mere 
local  tract  where  a  slight  tremor  has  been  experienced,  up 
to  such  catastrophes  as  that  of  Lisbon  in  1755,  which,  be- 
sides convulsing  the  Portuguese  coasts,  extended  into  the 
north  of  Africa  on  the  one  hand  and  to  Scandinavia  on 
the  other,  and  was  even  felt  as  far  as  the  east  of  North 
America.  Humboldt  computed  that  the  area  shaken  by 
this  great  earthquake  was  four  times  greater  than  that  of 
the  whole  of  Europe.  The  South  American  earthquakes 
are  remarkable  for  the  great  distances  to  which  their  effects 
extend  in  a  linear  direction.  Thus  the  strip  of  country  in 
Peru  and  Ecuador  severely  shaken  by  the  earthquake  of 
1868  had  a  length  of  2000  miles. 

Depth  of  source* — According  to  Mallet's  observations, 
over  the  centre  of  origin  the  shock  is  felt  as  a  vertical 
up-and-down  movement  (Seismic  vertical);  while,  receding 
from  this  centre  in  any  direction,  it  is  felt  as  an  undulatory 
movement,  and  comes  up  more  and  more  obliquely.  The 
angle  of  emergence,  as  he  termed  it,  was  obtained  by  him  by 
taking  the  mean  of  observations  of  the  rents  and  displace- 
ments of  walls  and  buildings.  In  Fig.  72,  for  example,  the 
wall  there  represented  has  been  rent  by  an  earthquake 
which  emerged  to  the  surface  in  the  path  marked  by  the 
arrow. 

By  observations  of  this  nature,  Mallet  estimated  the  ap- 


118  For  a  list  of  Peruvian  earthquakes  from  A.D.  1570  to  1875,  see  Geograph. 
Mag.  iv.  (1877),  p.  206.  The  earthquake  of  9  May,  1877,  at  Iquique,  and  its 
ocean -wave  are  described  by  E.  Geinitz,  Nova  Act!  Ac.  Cses.  Leopold.  Car.  xl. 
(1878),  pp.  383-444. 


DYNAMICAL    GEOLOGY 


467 


proximate  depth  of  origin  of  an  earthquake.  Let  Fig.  73, 
for  example,  represent  a  portion  of  the  earth's  crust  in 
which  at  a  an  earthquake  arises.  The  wave  of  shock  will 
travel  outward  in  successive  spherical  shells.  At  the  point 


Pig.  78.— Wall  shattered  by  an  Earthquake,  of  which  the  "path  of  emergence" 
has  been  in  the  direction  shown  by  the  arrow.    (After  Mallet.) 

e  it  will  be  felt  as  a  vertical  movement  and  loose  objects, 
such  as  paving-stones,  may  be  jerked  up  into  the  air,  and 
descend  bottom  uppermost  on  their  previous  sites.  At  c?, 
however,  the  wave  will  emerge  at  a  lower  angle,  and  will 
give  rise  to  an  undulation  of  the  ground,  and  the  oscillation 
of  objects  projecting  above  the  surface.  In  rent  buildings, 


Fig.  73.— Mallet's  mode  of  estimation  of  depth  of  source  of  Earthquake  movements. 

the  fissures  will  be  on  the  whole  perpendicular  to  the  path 
of  emergence.  By  a  series  of  observations  made  at  different 
points,  as  at  g  and/,  a  number  of  angles  are  obtained,  and 
the  point  where  the  various  lines  cut  the  vertical  (a)  will 


468  TEXT-BOOK    OF   GEOLOGY 

mark  the  area  of  origin  of  the  shock.  By  this  means, 
Mallet  computed  that  the  depth  at  which  the  impulse  of 
the  Oalabrian  earthquake  pf  1857  was  given  was  about  five 
miles.  As  the  general  result  of  his  inquiries,  he  concluded 
that,  on  the  whole,  the  origin  of  earthquakes  must  be  sought 
in  comparatively  superficial  parts  of  the  crust,  probably 
never  exceeding  a  depth  of  30  geographical  miles.  Follow- 
ing another  method  of  calculation,  Von  Seebach  computed 
that  the  earthquake  which  affected  Central  Europe  in  1872 
originated  at  a  depth  of  9'6  geographical  miles;  that  of 
Belluno  in  the  same  year  was  estimated  by  Hofer  to  have 
had  its  source  rather  more  than  4  miles  deep;  while  that 
of  Herzogenrath  in  1873  was  placed  by  Von  Lasaulx  at  a 
depth  of  about  14i  miles,  and  that  of  1877  in  the  same 
region  at  about  14  miles.179 

Geological  Effects* — These  are  dependent  not  only  on 
the  strength  of  the  concussion  but  on  the  structure  of  the 
ground,  and  on  the  site  of  the  disturbance,  whether  under- 
neath land  or  sea.  They  include  changes  superinduced  on 
the  surface  of  the  land,  on  terrestrial  and  oceanic  waters, 
and  on  the  relative  levels  of  land  and  sea. 

1.  Effects  upon  the  soil  and  general  sur- 
face of  a  country. — The  earth-wave  or  wave  of  shock 
underneath  a  country  may  traverse  a  wide  region  and 
affect  it  violently  at  the  time,  without  leaving  permanent 
traces  of  its  passage.  Blocks  of  rock,  however,  already 
disengaged  from  their  parent  masses  on  declivities,  may 
be  rolled  down  into  the  valleys.  Landslips  are  produced, 


119  See  papers  by  Hofer  and  A.  von  Lasaulx,  cited  on  p.  463.  For  an  account 
of  the  various  methods  employed  in  estimating  the  depth  of  origin  of  earth- 
quakes, see  Milne's  "Earthquakes,"  chapters  x.  and  xi.  Consult  also  the 
Trans.  Seismolog.  Soc.  Japan. 


DYNAMICAL    GEOLOGY  4o9 

which  may  give  rise  to  considerable  subsequent  changes 
of  drainage.  In  some  instances,  the  surfaces  of  solid  rocks 
are  shattered  as  if  by  gunpowder,  as  was  particularly 
noticed  to  have  taken  place  among  the  Primary  rocks  in 
the  Concepcion  earthquake  of  1835. 18°  It  has  often  been 
observed  also  that  the  soil  is  rent  by  fissures  which  vary 
in  size1  from  mere  cracks,  like  those  due  to  desiccation,  up 
to  chasms  a  mile  or  more  in  length  and  200  feet  or  more 
in  depth.  Permanent  modifications  of  the  landscape  may 
thus  be  produced.  Trees  are  thrown  down,  and  buried, 
wholly  or  in  part,  in  the  rents.  These  superficial  effects 
may,  indeed,  be  soon  effaced  by  the  levelling  power  of  the 
atmosphere.  Where,  however,  the  chasms  are  wide  and 
deep  enough  to  intercept  rivulets,  or  to  serve  as  channels 
for  heavy  rain -torrents,  they  are  sometimes  further  exca- 
vated, so  as  to  become  gradually  enlarged  into  ravines  and 
valleys,  as  has  happened  in  the  case  of  rents  caused  by 
the  earthquakes  of  1811-12  in  the  Mississippi  Valley.  In 
the  earthquake  which  shook  the  South  Island  of  New  Zea- 
land in  1848,  a  fissure  was  formed,  averaging  18  inches  in 
width  and  traceable  for  a  distance  of  60  miles  parallel 
to  the  axis  of  the  adjacent  mountain-chain.  The  subse- 
quent earthquake  of  1855,  in  the  same  region,  gave  rise 
to  a  fracture  which  could  be  traced  along  the  base  of  a 
line  of  cliff  for  a  distance  of  about  90  miles.  Dr.  Oldham 
has  described  a  remarkable  series  of  fissurings  which  ran 
parallel  with  the  river  of  Calhar,  Eastern  British  India, 
varying  with  it  to  every  point  of  the  compass  and  trace- 
able for  100  miles.181  The  great  Japanese  earthquake  of 


80  Darwin,  "Journal  of  Researches,"  1845,  p.  303. 

181  Q.  J.  Geol.  Soc.  xxviii.  p.  257.     For  a  catalogue  of  Indian  Earthquakes 
down  to  the  end  of  1869,  see  T.  Oldham,  Mem.  Geol.  Surv.  India,  xix.  part  2. 


470 


TEXT-BOOK    OF    GEOLOGY 


28th  October,  1891,  gave  rise  to  some  remarkable  fractures 
of  the  ground,  in  one  of  which  one  side  was  placed  per- 
manently at  a  different  level  from  the  other  (Fig.  74). 

Eemarkable  circular  cavities  have  been  noticed  in  Cala- 
bria and  elsewhere,  formed  in  the  ground  during  the  passage 
of  the  earth-wave.  In  many  cases,  these  holes  serve  as  fun- 
nels of  escape  for  an  abunbant  discharge  of  water,  so  that 


Fig.  74.— Fissure  or  fault  caused  by  the  earthquake  of  28th  October,  1891, 
in  the  Neo  Valley,  Japan. 

when  the  disturbance  ceases  they  appear  as  pools.  They 
are  believed  to  be  caused  by  the  sudden  collapse  of  subter- 
ranean water-channels  and  the  consequent  forcible  ejection 
of  the  water  to  the  surface.  Besides  water,  discharges  of 
various  gases  and  vapors,  sometimes  combustible,  have  been 
noted  at  the  fissures  formed  during  earthquakes. 

2.  Effects  upon  ter  res  tri  al  waters.189 — Springs 


Kluge,  Neues  Jahrb.  1861,  p.  777. 


DYNAMICAL    GEOLOGY  471 

are  temporarily  affected  by  earthquake  movements,  becom- 
ing greater  or  smaller  in  volume,  sometimes  muddy  or  dis- 
colored, and  sometimes  increasing  in  temperature.  Brooks 
and  rivers  have  been  observed  to  flow  with  an  interrupted 
course,  increasing  or  diminishing  in  size,  stopping  in  their 
flow  so  as  to  leave  their  channels  dry,  and  then  rolling  for- 
ward with  increased  rapidity.  Lakes  are  still  more  sensi- 
tive. Their  waters  occasionally  rise  and  fall  for  several 
hours,  even  at  a  distance  of  many  hundred  miles  from  the 
centre  of  disturbance.  Thus,  on  the  day  of  the  great  Lisbon 
earthquake,  many  of  the  lakes  of  central  and  northwestern 
Europe  were  so  affected  as  to  maintain  a  succession  of 
waves  rising  to  a  height  of  2  or  3  feet  above  their  usual 
level.  Cases,  however,  have  been  observed  where,  owing 
to  excessive  subterranean  movement,  lakes  have  been 
emptied  of  their  contents  and  their  beds  have  been  left 
permanently  dry.  On  the  other  hand,  areas  of  dry  ground 
have  been  depressed,  and  have  become  the  sites  of  new 
lakes. 

Some  of  the  most  important  changes  in  the  fresh  water 
of  a  region,  however,  are  produced  by  the  fall  of  masses  of 
rock  and  earth,  which,  by  damming  up  a  stream,  may  so 
arrest  its  water  as  to  form  a  lake.  If  the  barrier  be  of  suffi- 
cient strength,  the  lake  will  be  permanent;  though,  from  the 
usually  loose,  incoherent  character  of  its  materials,  the  dam 
thrown  across  the  pathway  of  a  stream  runs  a  great  risk  of 
being  undermined  by  the  percolating  water.  A  sudden  giv- 
ing way  of  the  barrier  allows  the  confined  water  to  rush 
with  great  violence  down  the  valley,  and  to  produce  per- 
haps tenfold  more  havoc  there  than  may  have  been  caused 
by  the  original  earthquake.  When  a  landslip  is  of  sufficient 
dimensions  to  divert  a  stream  from  its  previous  course,  the 


472  TEXT-BOOK    OF   GEOLOGY 

new  channel  thus  taken  may  become  permanent,  and  a  val- 
ley may  be  cut  out  or  widened. 

3.  Effects    upon    the    sea. —The    great    sea-wave 
propagated  outward  from  the  centre  of  a  sub-oceanic  earth- 
quake and  reaching  the  land  after  the  earth-wave  has  ar- 
rived there,  gives  rise  to  much  destruction  along  the  mari- 
time parts  of  the  disturbed  region.     When  it  approaches  a 
low  shore,  the  littoral  waters  retreat  seaward,  sucked  up,  as 
it  were,  by  the  advancing  wall  of  water,  which,  reaching  a 
height  of  sometimes  60  feet  or  more,  rushes  over  the  bare 
beach  and  sweeps  inland,  carrying  with  it  everything  which 
it  can  dislodge  and  bear  away.     Loose  blocks  of  rock  are 
thus  lifted  to  a  considerable  distance  from  their  former  posi- 
tion, and  left  at  a  higher  level.     Deposits  of  sand,  gravel, 
and  other  superficial  accumulations  are  torn  up  and  swept 
away,  while  the  surface  of  the  country,  as  far  as  the  limit 
reached  by  the  wave,  is  strewn  with  debris.     If  the  district 
has  been  already  shattered  by  the  passage  of  the  earth- wave, 
the  advent  of  the  great  sea-wave  augments  and  completes 
the  devastation.     The  havoc  caused  by  the  Lisbon  earth- 
quake of  1755,  and  by  that  of  Peru  and  Ecuador  in  1868, 
was  much  aggravated  by  the  co-operation  of  the  oceanic 
wave.     Where  the  wave  breaks  on  land  rising  out  of  deep 
water  little  damage  may  be  done. 

4.  Permanent   changes   of   level. — It   has    been 
observed,    after    the    passage   of   an   earthquake,    that  the 
level  of  the  disturbed  country  has  sometimes  been  changed. 
Thus  after  the  terrible  earthquake  of  19th  November,  1822, 
the  coast  of  Chile,  for  a  long  distance,  was  found  to  have 
risen  from  3  to  4  feet,  so  that  along  shore,  littoral  shells 
were  exposed  still  adhering  to  the  rocks,  amid  multitudes 
of  dead  fish.     The  same  coast-line  has  been  further  upraised 


DYNAMICAL    GEOLOGY  473 

by  subsequent  earthquake  shocks.  On  the  other  hand, 
many  instances  have  been  observed  where  the  effect  of  an 
earthquake  has  been  to  depress  permanently  the  disturbed 
ground.  For  example,  by  the  Bengal  earthquake  of  1762, 
an  area  of  60  square  miles  on  the  coast  near  Chittagong  sud- 
denly went  down  beneath  the  sea,  leaving  only  the  tops  of 
the  higher  eminences  above  water.  The  succession  of  earth- 
quakes which  in  the  years  1811  and  1812  devastated  the 
basin  of  the  Mississippi,  gave  rise  to  widespread  depressions 
of  the  ground,  over  some  of  which,  above  alluded  to,  the 
river  spread  so  as  to  form  new  lakes,  with  the  tops  of  the 
trees  still  standing  above  the  surface  of  the  water. 

Distribution  of  Earthquakes,183 — While  no  large  space  of 
the  earth's  surface  seems  to  be  free  from  at  least  some  de- 
gree of  earthquake-movement,  there  are  regions  more  es- 
pecially liable  to  the  visitation.  As  a  rule,  earthquakes  are 
most  frequent  in  volcanic  districts,  the  explosions  of  a  vol- 
cano being  generally  preceded  or  accompanied  by  tremors  of 
greater  or  less  intensity.  In  the  Old  "World,  a  great  belt  of 
earthquake  disturbance  stretches  in  an  east  and  west  direc-1 
tion,  along  that  tract  of  remarkable  depressions  and  eleva- 


188  For  European  earthquakes  an  alphabetical  catalogue  has  been  compiled 
by  Prof.  O'Reilly,  Trans.  Roy.  Irish  Academy,  xxviii.  (1886),  p.  489.  Cata- 
logue of  British  earthquakes,  op.  cit.  xxviii.  (1884),  p.  285.  C.  Davidson,  Geol. 
Mag.  1891,  p.  450.  Quart.  Journ.  Geol.  Soc.  xlvii.  (1891),  p.  618.  Detailed 
observations  of  the  effects  of  some  recent  European  earthquakes  will  be  found 
in  the  following  Memoirs.  The  Andalusian  earthquake  of  25th  Dec.  1884,  T. 
Taramelli  and  G.  Mercalli,  Real.  Accad.  Lincei,  1885-86,  p.  116,  Hubert,  Compt. 
Rend.  1885,  Fouque",  ibid.  20th  April,  1885,  and  the  large  quarto  volume  of  re- 
ports by  the  mission  specially  sent  to  study  the  phenomena  of  this  earthquake, 
Memoires  Acad.  Sci.  1889;  the  Ligurian  earthquake  of  23d  Feb.  1887,  T.  Tara- 
melli and  G.  Mercalli,  Ann.  Ufficio  Centrale  Meteorolog.  Geodinam.  part  iv.  vol. 
viii.  (1888),  Real.  Accad.  Lincei,  iv.  (1888);  the  Agram  earthquake  of  9th  Nov. 
1880,  "Grundzuge  der  Abyssodynamik, "  etc.,  by  S.  Pilar,  Agram,  1881;  the 
middle  German  earthquake  of  6th  March,  1872,  "Das  Mitteldeutsche  Erdbeben 
von  6  Marz,  1872,"  by  K.  von  Seebach,  Leipzig,  1873.  See  also  the  papers 
cited  on  pp.  459-465. 


474  TEXT-BOOK   OF   GEOLOGY 

tions  lying  between  the  Alps  and  the  mountains  of  northern 
Africa,  and  spreading  eastward  so  as  to  inclose  the  basins  of 
the  Mediterranean,  Black  Sea,  Caspian,  and  Sea  of  Aral, 
and  to  rise  into  the  great  inountain-ridges  of  Central  Asia. 
In  this  zone  lie  numerous  volcanic  vents,  both  active  and 
extinct  or  dormant,  from  the  Azores  on  the  west  to  the  basal- 
tic plateaus  of  India  on  the  east.  The  Pacific  Ocean,  sur- 
rounded with  a  vast  ring  of  volcanic  vents,  has  its  borders 
likewise  subject  to  frequent  earthquake  shocks.  Some  of 
the  most  terrible  earthquakes  within  human  experience  have 
been  those  which  have  affected  the  western  seaboard  of 
South  America.1"  It  is  worthy  of  notice  that  the  coasts 
of  the  Pacific  Ocean  more  specially  liable  to  convulsions  of 
this  nature  plunge  steeply  down  into  deep  water  with  slopes 
of  one  in  twenty  to  one  in  thirty,  while  shore-lines  such  as 
those  of  Australia,  Scandinavia,  and  the  east  of  South  Amer- 
ica, where  the  slope  is  no  more  than  from  one  in  fifty  to  one 
in  two  hundred  and  fifty,  are  hardly  ever  affected  by  earth- 
quakes. It  should  also  be  remarked  that  while  earthquakes 
are  apt  to  occur  along  the  flanks  of  mountain-chains  and  to 
travel  along  these  lines  of  elevation,  they  seldom  cross  a 
large  mountain-chain.  In  some  regions  the  site  of  disturb- 
ance is  not  on  land  but  under  the  sea.  This  has  been  clearly 
established  for  Japan.186 

Origin  of  Earthquakes.— Though  the  phenomena  of  an 
earthquake  become  intelligible  as  the  results  of  the  trans- 
mission of  waves  of  shock  arising  from  a  centre  where  some 


184  The  Charleston  Earthquake  of   31st  August,  1886,  has  been   fully  dis- 
cussed by  Captain  Button,  Ninth  Ann.  Report  U.  S.  Geol.  Survey,  1887-88,  p. 
209.     The  earthquakes  of  Central  America  are  discussed  by  F.  de  Montesstis 
de  Ballore  in  a  Memoir  rewarded  by  the  Acad.  Sci.  Nat.  Saone  et  Loire,  and 
published  at  Dijon,  1888. 

185  Milne,  "Earthquakes,"  p.  227. 


DYNAMICAL    GEOLOGY  475 

sudden  and  violent  impulse  has  been  given  within  the  ter- 
restrial crust,  the  origin  of  this  sudden  blow  can  only  be 
conjectured.  Various  conceivable  causes  may,  at  different 
times  and  under  different  conditions,  communicate  a  shock 
to  the  subterranean  regions.  Such  are  the  sudden  flashing 
into  steam  of  water  in  the  spheroidal  state,  the  sudden  con- 
densation of  steam,  the  explosions  of  a  volcanic  orifice,  the 
falling  in  of  the  roof  of  a  subterranean  cavity,  or  the  sudden 
snap  of  deep-seated  rocks  subjected  to  prolonged  and  in- 
tense strain. 

In  volcanic  regions,  the  frequent  earthquakes  which  pre- 
cede or  accompany  eruptions  are  doubtless  traceable  to  ex- 
plosions of  elastic  vapors,  and  notably  of  steam.  As  earth- 
~quakes  originate  also  in  districts  remote  from  any  active 
volcano,  and,  so  far  as  observation  shows,  at  comparatively 
shallow  depths,  these  cannot  be  connected  with  ordinary 
volcanic  action,  though  it  is  possible  that  by  movements  of 
molten  or  highly-heated  matter  within  the  crust  and  its  in- 
vasion of  the  upper  layer,  to  which  meteoric  water  in  con- 
siderable quantities  descends,  sudden  and  extensive  genera- 
tion of  steam  may  occasionally  take  place.18'  In  minor 
cases,  where  the  tremor  is  comparatively  slight  and  local, 
we  may  conceive  that  the  collapse  of  the  roof  or  sides  of 
some  of  the  numerous  tunnels  and  caverns  dissolved  out  of 
underground  rocks  by  permeating  water  may  suffice  to  pro- 
duce the  observed  shocks.187  The  copious  discharge  of  ma- 
terials from  a  volcanic  vent  may  produce  a  cavity  within  the 

88  Pfaff,  "Allgemeiue  Geologic  als  exacte  Wissenschaft,"  p.  230. 
181  In  the  Visp  Thai,  Canton  Wallis,  for  example,  where  there  are  some 
twenty  springs  carrying  up  gypsum  in  solution  (one  of  them  to  the  extent  of 
200  cubic  metres  annually),  continued  rumblings  and  sharp  shocks  are  from 
time  to  time  experienced.  In  July  and  August,  1855,  these  movements  lasted 
upward  of  a  month,  and  gave  rise  to  the  ftssuring  of  buildings  and  the  precipita- 
tion of  landslips.  In  the  honeycombed  limestone  tract  of  the  Karst,  also,  earth- 
quakes of  varied  intensity  are  of  constant  occurrence. 


•i76  TEXT-BOOK    OF   GEOLOGY 

earth,  the  crushing  in  of  which  will  give  rise  to  earthquakes. 
There  appears  reason  to  believe  that  the  most  convulsive 
earthquakes  originate  under  the  sea,  as  in  the  cases  of  the 
great  Lisbon  earthquake  and  those  of  Peru,  Chile,  and 
Japan.  For  these  it  is  as  yet  difficult  to  imagine  an  ade- 
quate cause.  Prof.  Milne  believes  that  they  may  be  partly 
due  to  disturbances  of  the  nature  of  volcanic  explosions, 
because  they  originate  beneath  the  sea,  and  the  vibrations 
have  the  peculiar  rapid  inward  motion  characteristic  of  the 
discharge  of  an  explosive  like  dynamite.168 

An  obvious  source  of  disturbance  within  the  earth  is  the 
rupture  of  rocfcs  within  the  crust  under  the  intense  strain 
produced  by  subsidence  upon  the  more  rapidly  contracting 
inner  hot  nucleus.  This  cause  may  conceivably  affect 
mountainous  areas;  but  we  do  not  know  how  it  would 
affect  the  sea-floor.  In  mountainous  districts,  many  dif- 
ferent degrees  of  shock,  from  mere  tremors  up  to  important 
earthquakes,  have  been  observed,  and  these  are  not  im- 
probably due  to  sudden  more  or  less  extensive  fractures  of 
rocks  still  under  great  strain.189  Hoernes,  from  a  study  of 
European  earthquake  phenomena,  concludes  that  though 
some  minor  earth-tremors  may  be  due  to  the  collapse  of 
underground  caverns,  and  others  of  local  character  to  vol- 
canic action,  the  greatest  and  most  important  earthquakes 
are  the  immediate  consequences  of  the  formation  of  moun- 
tains, and  he  connects  the  lines  followed  by  earthquakes 
with  the  structural  lines  of  mountain-axes.190 

From  what  was  stated  at  the  beginning  of  the  present  sec- 
tion, it  is  evident  that  where  the  earth's  crust  in  any  region 


iss  "Earthquakes,"  p.  281. 

189  See  postea,  p.  630.     Suess,  "Entstekung  der  Alpen."  Vienna,  1875. 

190  "Erdbeben  Studien,"  Jahrb.  Geol.  Reichs.  xxviii.  (1878),  p.  448. 


DYNAMICAL   GEOLOGY  477 

is  in  a  critical  condition  of  equilibrium,  some  connection 
may  be  expected  to  be  traceable  between  the  frequency  of 
earthquakes  and  the  earth's  position  with  regard  to  the  moon 
and  sun,  on  the  one  hand,  and  changes  of  atmospheric  condi- 
tions, on  the  other.  A  comparison  of  the  dates  of  recorded 
earthquakes  seems  to  bear  out  the  following  conclusions: 
1st.  An  earthquake  maximum  occurs  about  the  time  of  new 
moon;  2d.  Another  maximum  appears  two  days  after  the 
first  quarter;  3d.  A  diminution  of  activitjr  occurs  about  the 
time  of  full  moon;  4th.  The  lowest  earthquake  minimum 
is  on  the  day  of  the  last  quarter.191  There  is  likewise  ob- 
servable a  seasonal  maximum  and  minimum,  earthquakes 
over  most  of  the  northern  hemisphere  occurring  most  fre- 
quently in  winter,  and  least  frequently  in  summer.1"  Out  of 
656  earthquakes  chronicled  in  France  up  to  the  year  1846, 
three-fifths  took  place  in  the  winter,  and  two-fifths  in  the 
summer  months.  In  Switzerland  they  have  been  observed 
to  be  about  three  times  more  numerous  in  winter  than  in 
summer.  The  same  fact  is  remarked  in  the  history  even  of 
the  slight  earthquakes  in  Britain.  A  daily  maximum  ap- 
pears to  occur  about  2.30  A.M.,  and  a  minimum  about  three- 
quarters  of  an  hour  after  noon.  No  connection  has  yet  been 
satisfactorily  established  between  the  occurrence  of  earth- 
quakes and  sun-spots.  The  greater  frequency  of  earthquakes 
in  winter  might  be  expected  to  indicate  a  relation  between 
their  occurrence  and  atmospheric  pressure,  and  possibly 
earthquakes  are  more  frequent  with  a  low  than  with  a  high 
barometer.  "* 


191  J.  F.  J.  Schmidt,  "Studien  fiber  Erdbeben,"  2d  ed.  (1879),  p.  18. 

m  Ibid.  p.  20.     See  the  works  of  Perrey  cited  on  p.  459. 

m  Schmidt,  op.  cit.  p.  23.  F.  Groger,  Neues  Jahrb.  1878,  p.  928.  There 
doea  not  appear  to  be  any  marked  connection  between  the  state  of  the  barometer 
and  the  occurrence  of  earthquakes  in  Japan— J.  Milne,  "Earthquakes,"  p.  268. 


478  TEXT-BOOK    OF    GEOLOGY 

Section  iii.    Secular  Upheaval  and  Depression 

Besides  scarcely  perceptible  tremors  and  more  or  less  vio- 
lent movements  due  to  earthquake-shocks,  the  crust  of  the 
earth  is  generally  believed  to  undergo  in  many  places  oscil- 
lations of  an  extremely  quiet  and  uniform  character,  some- 
times in  an  upward,  sometimes  in  a  downward  direction. 
So  tranquil  may  these  changes  be,  as  to  produce  from  day 
to  day  no  appreciable  alteration  in  the  aspect  of  the  ground 
affected,  so  that  only  after  the  lapse  of  several  generations, 
and  by  means  of  careful  measurements,  can  they  really  be 
proved.  Indeed,  in  the  interior  of  a  country  nothing  but  a 
series  of  accurate  levellings  from  some  unmoved  datum-line 
might  detect  the  change  of  level,  unless  the  effects  of  the 
terrestrial  disturbance  showed  themselves  in  altering  the 
drainage.  Only  along  the  sea-coast  is  a  ready  measure 
afforded  of  any  such  movement. 

It  is  customary  in  popular  language  to  speak  of  the  sea 
rising  or  falling  relatively  to  the  land.  We  cannot  conceive 
of  any  possible  augmentation  of  the  oceanic  waters,  nor  of 
any  diminution,  save  what  may  be  due  to  the  extremely  slow 
processes  of  abstraction  by  the  hydration  of  minerals  and 
absorption  into  the  earth's  interior.  Any  changes,  there- 
fore, in  the  relative  levels  of  sea  and  land  must  be  due  to 
some  readjustment  in  the  form  either  of  the  solid  globe  or  of 
its  watery  envelope  or  of  both.  Play  fair  argued  at  the  be- 
ginning of  this  century  that  no  subsidence  of  the  sea-level 
could  be  local,  but  must  extend  over  the  globe.1'4  But  it  is 
now  recognized  that  what  is  called  the  sea-level  cannot  pos- 


m  "Illustrations  of  the  Hmtonian  Theory,"  1802.     The  same  conclusion 
was  announced  by  L.  von  Buch,  "Reise  durch  Norwegen  und  Lapland,"  1810. 


DYNAMICAL    GEOLOGY  479 

sess  the  uniformity  formerly  attributed  to  it;  that  on  the 
contrary  it  must  be  liable  to  local  distortion  from  the  attrac- 
tive influence  of  the  land.  Not  only  so,  but  the  level  of  the 
surface  of  large  inland  sheets  of  water  must  be  affected  by 
the  surrounding  high  lands. 

Mr.  E.  S.  Woodward,  whose  recent  memoir  on  this  sub- 
ject has  been  cited  (p.  68),  has  calculated  that  in  a  lake  140 
miles  broad  and  1000  feet  deep  in  the  middle,  the  difference 
of  level  of  the  water-surface  at  the  centre  and  at  the  margin 
may  amount  to  between  three  and  four  feet."6  As  already 
stated  he  has  further  computed  that  the  effect  of  the  conti- 
nents of  Europe  and  Asia  at  the  centre  in  disturbing  the 
sea-level  must  amount  to  about  2900  feet,  if  we  suppose  that 
there  is  no  deficiency  of  density  underneath  the  continent, 
and  to  only  about  10  feet  if  we  suppose  that  the  very  exist- 
ence of  the  continent  implies  such  a  deficiency. "' 

Various  suggestions  have  been  made  regarding  possible 
causes  of  alteration  of  the  sea-level.  (1)  A  shifting  of  the 
present  distribution  of  density  within  the  nucleus  of  the 
planet  would  affect  the  position  and  level  of  the  oceans  (ante, 
p.  89).  (2)  As  permanent  snow  and  ice  represent  so  much 
removed  from  the  general  body  of  water  on  the  globe,  any 
large  increase  or  diminution  in  the  extent  and  thickness  of 
the  polar  ice-caps  must  cause  a  corresponding  variation  in 
the  sea-level  (ante,  p.  44).  (3)  A  change  in  the  earth's  cen- 
tre of  gravity,  such  as  might  result  from  the  accumulation 
of  large  masses  of  snow  and  ice  as  an  ice-cap  at  one  of  the 
poles,  has  been  already  referred  to  (p.  43)  as  tending  to  raise 
the  level  of  the  ocean  in  the  hemisphere  so  affected,  and  to 


196  Bull.  U.  S.  Geol.  Surv.  No.  48  (1888),  p.  59. 

196  Op.  cit.  p.  85.     See  Stokes,  Trans.  Camb.  Phil.  Soc.  viii.  (1849),  p.  672; 
Sci.  Proc.  Roy.  Dublin  Soc.  v.  (1887),  p.  652. 


480  TEXT- BOOK    OF   GEOLOGY 

diminish  it  in  a  corresponding  measure  elsewhere.  The  re- 
turn of  the  ice  into  the  state  of  water  would  produce  an  op- 
posite effect.  The  attractive  influence  of  the  ice-sheets  of 
the  Glacial  Period  upon  the  sea-level  over  the  northern 
hemisphere  has  been  discussed  by  various  mathematicians, 
especially  by  Croll,  Pratt,  Heath,  and  Lord  Kelvin.  Con- 
siderable differences  appear  in  their  results,  according  to 
the  conditions  which  they  postulate,  but  they  agree  that  a 
decided  elevation  of  the  sea-level  must  be  attributed  to  the 
accumulation  of  thick  masses  of  snow  and  ice.  The  rise  of 
the  sea-level  along  the  border  of  an  ice-cap  of  38°  angular 
radius  and  10,000  feet  thick  in  the  centre  is  estimated  at 
from  139  to  573  feet.1*7  (4)  A  still  further  conceivable 
source  of  geographical  disturbance  ist  to  be  found  in  the 
fact  that,  as  a  consequence  of  the  diminution  of  centrifugal 
force  owing  to  the  retardation  of  the  earth's  rotation  caused 
by  the  tidal  wave,  the  sea-level  must  have  a  tendency  to 
subside  at  the  equator  and  rise  at  the  poles."9  A  larger 
amount  of  land,  however,  need  not  ultimately  be  laid  bare 
at  the  equator,  for  the  change  of  level  resulting  from  thig 
cause  would  be  so  slow  that,  as  Dr.  Croll  has  pointed  out, 


i"  See  Croll,  "Climate  and  Time,"  chaps,  xxiii.,  xxiv.  Geol.  Mag,  1874. 
Pratt,  "Figure  of  the  Earth,"  D.  D.  Heath,  Phil.  Mag.  xxxi.  (1866),  pp.  201, 
323,  xxxii.  (1866),  p.  34.  Thomson  (Lord  Kelvin),  op.  cit.  xxxi.  p.  305.  A. 
Penck,  Jahrb.  Geograph.  Gesel.  Munich,  vii.  De  Lapparent,  Bull.  Soc.  Geol. 
France,  xiv.  1886,  p.  368,  Revue  Generate  des  Sciences,  May,  1890.  R.  S. 
Woodward,  Bull.  U.  S.  Geol.  Survey,  No.  48.  Von  Drygalski,  "Bewegungen 
der  Kontinente  zur  Eiszeit,"  Berlin,  1889.  Prof.  Suess  believes  that  the  limits 
of  the  dry  land  depend  upon  certain  large  indeterminate  oscillations  of  the  stati- 
cal figure  of  the  oceanic  envelope;  that  not  only  are  "raised  beaches"  to  be  thus 
explained,  but  that  there  are  absolutely  no  vertical  movements  of  the  crust  save 
such  as  may  form  part  of  the  plication  arising  from  secular  contraction ;  and 
that  the  doctrine  of  secular  fluctuations  in  the  level  of  the  continents  is  merely 
a  remnant  of  the  old  "Erhebungstheorie,"  destined  to  speedy  extinction.  "Ant- 
lit/  der  Erde,"  Leipzig,  1883.  Pfaff  defends  the  general  opinion  against  these 
views  in  Zeitsch.  Deutsch.  Geol.  Ges.  1884. 

198  Croll,  Phil.  Mag.  1868,  p.  382.  Thomson,  Trans.  Geol.  Soc.  Glasgow, 
iii.  p.  223. 


DYNAMICAL    GEOLOGY  481 

the  general  degradation  of  the  surface  of  the  land  might 
keep  pace  with  it  and  diminish  the  terrestrial  area  as  much 
as  the  retreat  of  the  ocean  tended  to  increase  it.  The  same 
writer  has  further  suggested  that  the  waste  of  the  equatorial 
land,  and  the  deposition  of  the  detritus  in  higher  latitudes, 
may  still  further  counteract  the  effects  of  retardation  and 
the  consequent  change  of  ocean-level.  (5)  Some  geologists 
have  supposed  that  where  the  earth's  crust  is  loaded  with 
thick  deposits  of  sediment  or  massive  ice-sheets  it  will  tend 
to  sink,  while  on  the  other  hand  denudation  by  unloading  it 
promotes  upheaval. 

The  balance  of  evidence  at  present  available  seems  ad- 
verse to  any  theory  which  would  account  for  ancient  and 
modern  changes  in  the  relative  level  of  sea  and  land  by 
variations  in  the  figure  of  the  oceanic  envelope,  save  to  a 
limited  extent  by  the  attraction  caused  by  extensive  masses 
of  upraised  land,  and  possibly  in  northern  and  southern  lati- 
tudes by  the  attractive  influence  of  large  accumulations  of 
snow  and  ice.  Such  changes  are  rather  to  be  regarded  as 
due  to  movements  of  the  solid  crust.  The  proofs  of  up- 
heaval and  subsidence,  though  sometimes  obtainable  from 
wide  areas,  are  marked  by  a  want  of  uniformity  and  a  local 
and  variable  character,  indicative  of  an  action  local  and  vari- 
able in  its  operations,  such  as  the  folding  of  the  terrestrial 
crust,  and  not  regular  and  widespread,  such  as  might  be 
predicated  of  any  alteration  of  sea-level.  While  admitting 
therefore  that,  to  a  certain  extent,  oscillations  of  the  rela- 
tive level  of  sea  and  land  may  have  arisen  from  some  of  the 
causes  above  enumerated,  we  may  hold  that,  on  the  whole, 
it  is  the  land  which  rises  and  sinks  rather  than  the  sea.1" 

1M  For  the  arguments  against  the  view  above  adopted  and  in  favor  of  the 
doctrine  that  the  increase  of  the  land  above  sea- level  is  due  to  the  retirement  of 
GEOLOGY— Vol.  XXIX— 21 


482  TEXT-BOOK   OF   GEOLOGY 

§  1.  Upheaval. — Various  maritime  tracts  of  land  have 
been  ascertained  to  have  undergone  in  recent  times,  or  to  be 
still  undergoing,  what  appears  to  be  a  gradual  elevation 
above  the  sea.  On  the  coast  of  Siberia,  for  600  miles  to 
the  east  of  the  river  Lena,  round  the  islands  of  Spitzbergen 
and  Novaja  Zemlja,  along  the  shores  of  the  Scandinavian 
peninsula  with  the  exception  of  a  small  area  at  its  southern 
apex,  and  along  a  maritime  strip  of  western  South  America, 
it  has  been  proved  that  the  sea  stands  now  at  a  lower  level 
with  regard  to  the  land  than  it  formerly  did.  In  searching 
for  proofs  of  such  movements  the  student  must  be  on  his 
guard  against  being  deceived  by  any  apparent  retreat  of  the 
sea,  which  may  be  due  merely  to  the  deposit  of  gravel,  sand, 
or  mud  along  the  shore,  and  the  consequent  gain  of  land. 
Local  accumulations  of  gravel,  or  "storm  beaches,"  are 
often  thrown  up  by  storms,  even  above  the  level  of  ordi- 
nary high-tide  mark.  In  estuaries,  also,  considerable  tracts 
of  low  ground  are  gradually  raised  above  the  tide-level  by 
the  slow  deposit  of  mud.  The  following  proofs  of  actual  rise 
of  the  land  are  chiefly  relied  on  by  geologists.800 

Evidence  from  dead  organisms. — Bocks  cov- 
ered with  barnacles  or  other  littoral  adherent  animals,  or 
pierced  by  lithodomous  shells,  afford  presumptive  proof 
of  the  presence  of  the  sea.  A  single  stone  with  these 
creatures  on  its  surface  would  not  be  satisfactory  evidence, 
for  it  might  have  been  cast  up  by  a  storm;  but  a  line  of 
large  bowlders,  which  had  evidently  not  been  moved  since 
the  cirripeds  and  mollusks  lived  upon  them,  and  still  more 
a  solid  cliff  with  these  marks  of  littoral  or  sub-littoral  life 


the  sea,  see  H.  Trautschold,  Bulletin  Societe  Imp.  des  Naturaliates  de  Moscou, 
xlii.  (1869),  part  i.  p.  1  ;  1883,  No.  2,  p.  341 ;  Bull.  Soc.  Geol.  France  (3),  viii. 
(1879),  p.  134;  but  more  especially  Sueas,  in  his  great  work  the  "Antlitz  der 
Erde." 

900  See  ''Earthquakes  and  Volcanoes"  (A.  G.),  Chambers's  Miscellany  of 
Tracts. 


DYNAMICAL    GEOLOGY  483 

upon  its  base,  now  raised  above  high-water  mark,  would  be 
sufficient  to  demonstrate  a  change  of  level.  The  amount 
of  this  change  might  be  pretty  accurately  determined  by 
measuring  the  vertical  distance  between  the  upper  edge  of 
the  barnacle  zone  upon  the  upraised  rock,  and  the  limit 
of  the  same  zone  on  the  present  shore.  By  this  kind  of 
evidence,  the  recent  uprise  of  the  coast  of  Scandinavia  has 
been  proved.  The  shell-borings  on  the  pillars  of  the  temple 
of  Jupiter  Serapis  in  the  Bay  of  Naples  prove  first  a  depres- 
sion and  then  an  elevation  of  the  ground  to  the  extent  of 
more  than  twenty  feet.201  Raised  coral-reefs,  formed  by 
living  species  of  corals,  are  a  conspicuous  feature  of  the 
geology  of  the  West  Indian  Region.  The  terraces  of  Bar- 
badoes  are  particularly  striking.  In  Cuba,  a  raised  coral- 
reef  occurs  at  a  height  of  1000  or  1100  feet  above  the  sea.108 
In  Peru,  modern  coral-limestone  has  been  found  2900  or 
3000  feet  above  sea-level.*08  Again,  in  the  Solomon  Islands, 
evidence  of  recent  uprise  is  furnished  by  coral-reefs  lying 
at  a  height  of  1100  feet,"04  and  similar  evidence  occurs  among 
the  New  Hebrides  at  1500  feet. 

The  elevation  of  the  sea-bottom  can  in  like  manner  be 
proved  by  dead  organisms  fixed  in  their  position  of  growth 
beneath  high-water  mark.  Thus  dead  specimens  of  Mya 
truncata  occur  on  some  parts  of  the  coast  of  the  Firth 
of  Forth  in  considerable  numbers,  still  placed  with  their 
siphuncular  end  uppermost  in  the  stiff  clay  in  which  they 
burrowed.  The  position  of  these  shells  is  about  high-water 
mark,  but  as  their  existing  descendants  do  not  live  above 
low-water  mark,  we  may  infer  that  the  coast  has  been  raised 
by  at  least  the  difference  between  high-  and  low-water 
mark,  or  eighteen  feet."0*  Dead  shells  of  the  large  Pholas 
dactylus  occur  in  a  similar  position  near  high-water  mark 
on  the  Ayrshire  coast.  Even  below  low-water,  examples 
have  been  noted,  as  in  the  interesting  case  observed  by  Sara 
on  the  Drobaksbank  in  the  Christiania  Fjord,  where  dead 
stems  of  Oculina  prolifera  (L.)  occur  at  depths  of  only  ten 
or  fifteen  fathoms.  This  coral  is  really  a  deep-sea  form, 
living  on  the  western  and  northern  coasts  of  Norway,  at 


m  Babbage,  Edin.  Phil.  Journ.  xi.  (1824),  p.  91.     J.  D.  Forbes,  Edin.  Journ. 

Sci.  i.  (1829),  p.  260.     Lyell,  "Principles,"  ii.  p.  164. 

ios  A.  Agassiz,  Amer.  Acad.  xi.  (1882),  p.  119. 

803  A.  Agassiz,  Bull.  Mus.  Comp.  Zool.  vol.  iii. 

904  H.  B.  Guppy,  Nature,  3d  Januarv,  1884. 

805  Hugh  Miller's  "Edinburgh  and  it's  Neighborhood,"  p.  110. 


484  TEXT-BOOK    OF   GEOLOGY 

depths  of  one  hundred  and  fifty  to  three  hundred  fathoms 
in  cold  water.  It  must  have  been  killed  as  the  elevation 
of  the  area  brought  it  up  into  upper  and  warmer  layers  of 
water."06  It  has  even  been  said  that  the  pines  on  the  edges 
of  the  Norwegian  snow-fields  are  dying  in  consequence  of 
the  secular  elevation  of  the  land  bringing  them  up  into 
colder  zones  of  the  atmosphere. 

Any  stratum  of  rock  containing  marine  organisms  which 
have  manifestly  lived  and  died  where  their  remains  now  lie, 
may  be  held  to  prove  a  change  of  level  between  sea  and 
land.  In  this  way  it  can  be  shown  that  most  of  the  solid 


Fig.  76.— View  of  a  line  of  ancient  sea-cliff  pierced  at  the  base  with  sea-worn 
caves  and  fronted  by  a  Raised  Beach. 

land  now  visible  to  us  has  once  been  under  the  sea.  High 
on  the  flanks  of  mountain-chains  (as  in  the  Alps  and  Hima- 
layas), undoubted  marine  shells  occur  in  the  solid  rocks. 

Sea-worn  Caves. — A  line  of  sea- worn  caves,  now 
standing  at  a  distance  above  high-water  mark  beyond  the 
reach  of  the  sea,  affords  evidence  of  recent  change  of  level. 
In  the  accompanying  diagram  (Fig.  75)  examples  of  such 
caves  are  seen  at  the  base  of  the  cliff,  once  the  sea-margin, 
now  separated  from  the  tide  by  a  platform  of  meadow- 
land. 

Raised  Beaches  furnish  one  of  the  most  striking 


206  Quoted  by  Vom  Rath  in  a  paper  entitled  "Aus  Norwegen,"  Neues  Jahrb. 
1869,  p.  422.  For  another  example,  see  G-wyn  Jeffreys,  Brit.  Assoc.  1867, 
p.  431. 


DYNAMICAL    GEOLOGY 


485 


proofs  of  change  of  level.  A  beach  or  space  between  tide- 
marks,  where  the  sea  is  constantly  grinding  down  sand  and 
gravel,  mingling  with  them  the  remains  of  shells  and  other 
organisms,  sometimes  piling  the  deposits  up,  sometimes 
sweeping  them  away  out  into  opener  water,  forms  a  famil- 
iar terrace  or  platform  on  coast- lines  skirting  tidal  seas. 
When  this  margin  of  littoral  deposits  has  been  placed 
above  the  reach  of  the  waves,  the  flat  terrace  thus  elevated 
is  known  as  a  "raised  beach"  (Figs.  75,  76,  77,  78).  The 
former  high- water  mark  then  lies  inland,  and  while  its  sea- 
worn  caves  are  in  time  hung  with  ferns  and  mosses,  the 
beach  across  which  the  tides  once  flowed  furnishes  a  plat- 
form on  which  meadows,  fields,  gardens,  roads,  houses, 
villages,  and  towns  spring  up,  while  a  new  beach  is  made 
below  the  margin  of  the  uplifted  one.  A  series  of  raised 
beaches  may  occur  at  various  heights  above  the  sea.  Each 
terrace  marks  a  former  lower  level 
of  the  land  with  regard  to  the  sea, 
and  probably  a  lengthened  stay  of 
the  land  at  that  level,  while  the  in- 
tervals between  them  represent  the 
vertical  amount  of  each  variation 
in  the  relative  levels  of  sea  and 
land,  and  show  that  the  interval 
between  the  changes  was  too  brief 
for  the  formation  of  terraces.  A 

Succession    of    raised  beaches,  rising  Fig.76. -Section  of  a  Raised  Beach 

above  the  present  sea-level,  may  ^S^0^S&^^ 
therefore  be  taken  as  pointing  to  a  y^SStSSSfff^ 
former  intermittent  upheaval  of  the  cay  of  abundant  iand-sheiis. 
country,  interrupted  by  long  pauses,  Fistra11  Bay' Cornwa11  (B')' 
during  which  the  general  level  did  not  materially  change, 
unless  in  regions  where  there  is  reason  to  believe  that  the 
surface  of  the  sea  has  undergone  a  change  of  level  from 
the  accumulation  or  melting  of  large  masses  of  snow  and 
ice  (ante,  p.  43). 

Raised  beaches  abound  in  the  higher  latitudes  of  the 
northern  and  southern  hemispheres,  and  this  distribution 
has  been  claimed  as  a  strong  argument  in  favor  of  the  view 
that  they  are  due  to  a  fall  of  the  local  level  of  the  sea- 
surface  from  the  disappearance  or  diminution  of  former 
ice-caps.  That  some  at  least  of  the  raised  beaches  in  these 
regions  may  be  due  to  this  cause  may  be  granted.  The 
gradual  rise  of  level  of  the  beaches  when  traced  up  the 
fjords,  which  has  been  repeatedly  asserted  for  some  dis- 


486  TEXT-BOOK    OF   GEOLOGY 

tricts,  would  be  the  natural  effect  of  the  greater  mass  of 
ice  in  the  interior.  In  the  exploration  of  the  lake  regions 
of  North  America  numerous  instances  have  been  described 
of  a  slope  upward  of  the  former  water-levels  toward  the 
main,  ice-fields.  A  remarkable  example  is  furnished  by 
the  terraces  of  the  vanished  glacial  sheet  of  water  callea 
Lake  Agassiz  which  once  filled  the  basin  of  the  Bed  Eiver 
of  the  North.  Mr.  Warren  Upham  has  found  that  these 
ancient  lines  of  water-level  gradually  rise  from  south  to 
north  and  from  west  to  east,  in  the  direction  of  the  former 
ice-fields,  the  amount  of  slope  ranging  from  zero  to  1-3  feet 
per  mile."01  Mr.  Gr.  K.  Gilbert  has  noticed  a  rise  of  as  much 
as  5  feet  in  a  mile  among  the  old  terraces  of  Lake  Ontario.1"" 
Raised  beaches  occur  round  many  parts  of  the  coast-line 
of  Britain.  De  la  Beche  gives  the  subjoined  view  (Fig.  77) 


Fig.  77.— View  of  Raised  Beach,  Nelly's  Cave,  Cornwall  (B.). 

of  a  Cornish  locality  where  the  existing  beach  is  flanked  by 
a  cliff  of  slate,  &,  continually  cut  away  by  the  sea  so  that 
the  overlying  raised  beach,  a,  c,  will  ere  long  disappear. 
The  coast  line  on  both  sides  of  Scotland  is  likewise  fringed 
with  raised  beaches,  sometimes  four  or  five  occurring  above 
each  other  at  heights  of  25,  40,  50,  60,  75,  and  100  feet 
above  the  present  high- water  mark.809  Others  are  found  on 
both  sides  of  the  English  Channel.810  The  sides  of  the 


*"  Bui].  U.  S.  Geol.  Surv.  No.  39  (1887),  pp.  18,  20. 

208  Science,  i.  p.  222. 

409  For  accounts  of  some  British  raised  beaches,  see  De  la  Beche,  Report 
on  Geology  of  Devon  and  Cornwall,"  chap.  xiii. ;  C.  Maclaren,  "Geology  of 
Fife  and  the  Lothians,"  1839;  R.  Chambers,  "Ancient  Sea  Margins";  Prest- 
wich,  Q.  J.  Geol.  Soc.  xxviii.  p.  38 ;  xxx.  p.  29 ;  R.  Russell  and  T.  Y.  Holmes, 
Brit.  Assoc.  1876,  Sects,  p.  95;  Ussher,  Geol.  Mag.  1879,  p.  166. 

"«  On  the  raised  beach  of  Sangatte,  near  Calais,  see  Prestwich,  Bull.  Soc. 


DYNAMICAL    GEOLOGY  4.87 

mountainous  fjords  of  Northern  Norway,  up  to  more  than 
600  feet  above  sea-level,  are  marked  with  conspicuous  lines 
of  terraces  (Fig.  78).  These  terraces  are  partly  ordinary 
beach  deposits,  partly  notches  cut  out  of  rock,  probably 
with  the  aid  of  drifting  coast-ice.811  Proofs  of  recent  eleva- 
tion of  the  shores  of  the  Mediterranean  are  furnished  by 
raised  beaches  at  various  heights  above  the  present  water- 
level.  In  Corsica  such  terraces  occur  at  heights  of  from 
15  to  20  metres.  "• 

On  the  west  coast  of  South  America,  lines  of  raised  ter- 
race containing  recent  shells  have  been  traced  by  Darwin 


Pig.  78.— View  of  Terraces,  Alten  Fjord,  Norway. 

as  proofs  of  a  great  upheaval  of  that  part  of  the  globe  in 
modern  geological  time.  The  terraces  are  not  quite  hori- 
zontal but  rise  toward  the  south.  On  the  frontier  of  Bolivia, 
they  occur  at  from  65  to  80  feet  above  the  existing  sea-level, 

Geol.  France  (3),  viii.  (1880),  p.  547 ;  on  those  of  Finisterre,  C.  Barroia,  Ann. 
Soc.  Geol.  Nord.  ix.  (1882). 

511  See  R.  Chambers,  "Tracings  of  the  North  of  Europe"  (1850),  p.  172  et  seq. 
Bravais,  "Voyages  de  la  Commission  Scientifique  du  Nord,"  etc.,  translated  in 
Q.  J.  Geol.  Soc.  i.  p.  534.  Kjerulf,  Z.  Deutsch.  Geol.  Ges.  xxii.  p.  1  ;  "Die 
Geologie  des  siid.  und  mittl.  Norwegen,"  1880,  p.  7;  Geol.  Mag.  viii.  p.  74. 
S.  A.  Sexe,  "On  Rise  of  Land  in  Scandinavia,"  Index  Scholarum  of  University, 
Christiania,  1872.  H.  Mohn.  Nyt.  Mag.  Nat.  xxii.  p.  1.  Dakyns,  Geol.  Mag. 
1877,  p.  72.  K.  Pettersen,  Arch.  Math.  Nat.  Christiania,  1878,  p.  182,  x.  (1885); 
Geol.  Mag.  1879,  p.  298;  Tromso  Museums  Aarshefter,  III.  1880.  Sitz.  Akad. 
Wien.  xcviii.  (1889).  Lehmann,  "Ueber-ehemalige  Strandliiiier, "  etc.,  Halle, 
1879;  Zeitsch.  ges.  Naturwiss.  1880,  p.  280.  A.  G.  Hogbom,  Geol.  For.  For- 
handl.  Stockholm,  ix.  1887,  p.  19.  C.  Sandier,  Petermann's  Mittheil.  xxxvi. 
>p.  209,  235. 
ull.  Soc.  Geol.  France  (3),  iv.  p.  86. 


438  TEXT-BOOK   OF   GEOLOGY 

but  nearer  the  higher  mass  of  the  Chilean  Andes  they  are 
found  at  1000,  and  near  Valparaiso  at  1300  feet.  That  some 
of  these  ancient  sea-margins  belong  to  the  human  period 
was  shown  by  Mr.  Darwin's  discovery  of  shells  with  bones 
of  birds,  ears  of  maize,  plaited  reeds  and  cotton  thread,  in 
one  of  the  terraces  opposite  Callao  at  a  height  of  85  feet.313 
Eaised  beaches  occur  in  New  Zealand,  and  indicate  a  greater 
change  of  level  in  the  southern  than  in  the  northern  part  of 
the  country.*14  It  should  be  observed  that  this  increased 
rise  of  the  terraces  poleward  occurs  both  in  the  northern 
and  southern  hemispheres,  and  is  one  of  the  chief  facts 
insisted  upon  by  those  who  would  explain  the  terraces  by 
displacements  of  the  sea  Tather  than  of  the  land. 

*Human  Records  and  Traditions. — In  countries 
which  have  been  long  settled  by  a  human  population,  it  is 
sometimes  possible  to  prove,  or  at  least  to  render  probable, 
the  fact  of  recent  change  of  level  by  reference  to  tradition, 
to  local  names,  and  to  works  of  human  construction.  Piers 
and  harbors,  if  now  found  to  stand  above  the  upper  limit 
of  high-water,  furnish  indeed  indisputable  evidence  of  a 
rise  of  land  or  fall  of  sea-level  since  tneir  erection.  Numer- 
ous proofs  of  a  recent  change  of  level  in  the  coast  of  the 
Arctic  Ocean  from  Spitzbergen  eastward  have  been  ob- 
served. The  Finnish  coast  is  reported  to  have  risen  6  feet 
4  inches  in  127  years.*1*  At  Spitzbergen  itself,  besides  its 
raised  beaches,  bearing  witness  to  previous  elevations,  small 
islands  which  existed  two  hundred  years  ago  are  now  joined 
to  larger  portions  of  land.  At  Novaja  Zemlja,  where  six 
raised  beaches  were  found  by  Nordenskiold,  the  highest 
being  600  feet  above  sea-level,918  there  seems  to  have  been 
a  rising  of  the  sea-bottom  to  the  extent  of  100  feet  or  more 
since  the  Dutch  expedition  of  1594.  On  the  north  coast 
of  Siberia  the  island  of  Diomida,  observed  in  1760  by 
Chalaourof  to  the  east  of  Cape  Sviatoj,  was  found  by 
Wrangel  sixty  years  afterward  to  have  been  united  to  the 
mainland.117  From  marks  made  on  the  coast  in  the  middle 


213  "Geological  Observations,"  chap.  x.     See  Geol.  Mag.  1877,  p.  28. 

214  Haast's  "Geology  of  Canterbury,"  1879,  p.  366. 

215  Nature,  xxvi.  p.  231.  816  Ibid.  xv.  p.  123. 

217  Grad,  Bull.  Soc.  Geol.  France,  3d  ser.  ii.  p.  348.  Traces  of  oscillations 
of  level  within  historic  times  have  been  observed  in  the  Netherlands,  Flanders 
and  Upper  Italy.  Bull.  Soc.  Geol.  France,  2d  ser.  xix.  p.  556;  3d  ser.  ii.  pp. 
46,  222 ;  Ann.  Soc.  Geol.  Nord.  v.  p.  218.  For  alleged  changes  of  level  in  the 
estuary  of  the  Garonne,  see  Artigues,  Act.  Soc.  Linu.  Bordeaux,  xxxi.  (1876), 
p.  287,  and  Delfortrie,  ib.  xxxii.  p.  79. 


DYNAMICAL    GEOLOGY  489 

of  last  century  it  appears  that  the  north  of  Sweden  has  risen 
about  7  feet  in  the  last  154  years,  but  that  the  movement 
has  lessened  southward  until  in  Scania  it  has  been  replaced 
by  one  in  a  downward  direction  (see  p.  493). 

§  2.  Subsidence. — It  is  more  difficult  to  trace  a  downward 
movement  of  land,  for  the  evidence  of  each  successive  sea- 
margin  is  carried  down  and  washed  away  or  covered  up. 
The  student  will  take  care  to  guard  himself  against  being 
misled  by  mere  proofs  of  the  advance  of  the  sea  on  the  land. 
In  the  great  majority  of  cases,  where  such  an  advance  is 
taking  place,  it  is  due  not  to  subsidence  of  the  land,  but  to 
erosion  of  the  shores.  It  is,  indeed,  the  converse  of  the 
deposition  above  mentioned  (p.  482)  as  liable  to  be  mistaken 
for  proof  of  upheaval.  The  results  of  mere  erosion  by  the 
sea,  however,  and  those  of  actual  depression  of  the  level  of 
the  land,  cannot  always  be  distinguished  without  some  care. 
The  encroachment  of  the  sea  upon  the  land  may  involve  the 
disappearance  of  successive  fields,  roads,  houses,  villages, 
and  even  whole  parishes,  without  any  actual  change  of  level 
of  the  land.  Certain  causes,  however,  referred  to  below, 
may  come  into  operation,  producing  an  actual  submergence 
of  land  without  any  real  subsidence  of  the  land  itself.  The 
following  kinds  of  evidence  are  usually  cited  to  prove  sub- 
sidence. 

Submerged  Forests. — As  the  land  is  brought 
within  reach  of  the  waves,  and  its  characteristic  surface- 
features  are  effaced,  the  submerged  area  may  retain  little  or 
no  evidence  of  its  having  been  a  land-surface.  It  will  be 
covered,  as  a  rule,  with  sea-worn  sand  or  silt.  Hence,  no 
doubt,  the  reason  why,  among  the  marine  strata  which  form 
so  much  of  the  stratified  portion  of  the  earth's  crust,  and 
contain  so  many  proofs  of  depression,  actual  traces  of  land- 
surfaces  are  comparatively  rare.  It  is  only  under  very 
favorable  circumstances,  as,  for  instance,  where  the  area 
is  sheltered  from  prevalent  winds  and  waves,  and  where, 


490  TEXT-BOOK    OF   GEOLOGY 

therefore,  the  surface  of  the  land  can  sink  tranquilly  under 
the  sea,  that  fragments  of  that  surface  may  be  preserved 
under  overlying  marine  accumulations.  It  is  iii  such  places 
that  "submerged  forests"  occur  (Fig.  79).  These  are  stumps 
of  trees  still  in  their  positions  of  growth  in  their  native  soil, 
often  associated  with  beds  of  peat,  full  of  tree-roots,  hazel- 
nuts,  branches,  leaves,  and  other  indications  of  a  terrestrial 
surface.  There  is  sometimes,  however,  considerable  risk  of 
deception  in  regard  to  the  nature  and  value  of  such  evidence 
of  depression.  Where,  for  instance,  shingle  or  sand  is 
banked  up  against  a  shore  or  river-mouth,  considerable 
spaces  may  be  inclosed  and  filled  with  fresh-water,  the  bot- 
tom of  which  may  be  some  way  below  high- water  mark.  In 
such  lagoons  terrestrial  vegetation  and  debris  from  the  land 

t^^^^J^s^yi^MM^ 


Fig.  79.— Section  of  Submerged  Forest  (B.). 

A  platform  of  older  rocks  (e  e)  has  been  covered  with  soil  (d  d)  on  which  trees  (a  a  a) 
have  established  themselves.  In  course  of  time,  after  some  of  the  trees  had 
fallen  (6),  and  a  quantity  of  vegetable  soil  had  accumulated,  inclosing  here  and 
there  the  bones  of  deer  and  oxen  (c  c),  the  area  sank,  and  the  sea  overflowing  it 
threw  down  upon  its  surface  saudy  or  muddy  deposits  (//). 

may  be  deposited.  Eventually,  if  the  protecting  barriers 
should  be  cut  away  the  tides  may  flow  over  the  layers  of 
terrestrial  peat,  giving  a  false  appearance  of  subsidence. 
Again,  owing  to  removal  of  subterranean  sandy  deposits  by 
springs,  overlying  peat-beds  may  sink  below  sea-level. 

De  la  Bee  he  has  described, 'round  the  shores  of  Devon, 
Cornwall,  and  western  Somerset,  a  vegetable  accumulation, 
consisting  of  plants  of  the  same  species  as  those  which  now 
grow  freely  on  the  adjoining  land,  and  occurring  as  a  bed  at 
the  mouths  of  valleys,  at  the  bottoms  of  sheltered  bays,  and 
in  front  of  and  under  low  tracts  of  land,  of  which  the  sea- 
ward side  dips  beneath  the  present  level  of  the  sea."*  Over 


*18  "Geolopy  of  Devon  and  Cornwall,"  Mem.  Gteol.   Survey.     For  further 
accounts  oi  -Britisii  submerged  forests,  see  Q.  J.  Geol.  iSoc.  ixii.  p.  1. ;  xxxiv. 


DYNAMICAL    GEOLOGY  491 

this  submerged  land-surface,  sand  and  silt  containing  estua- 
rine  shells  have  generally  been  deposited,  whence  we  may 
infer  that,  in  the  submergence,  the  valleys  first  became 
estuaries,  and  then  sea-bays.  If  now,  in  the  course  of  ages, 
a  series  of  such  submerged  forests  should  be  formed  one 
over  the  other,  and  if,  finally,  they  should,  by  upheaval  of 
the  sea-bottom,  be  once  more  laid  dry,  so  as  to  Ibe  capable 
of  examination  by  boring,  well-sinking,  or  otherwise,  they 
would  prove  a  former  long-continued  depression,  with  inter- 
vals or  rest.  These  intervals  would  be  marked  by  the 
buried  forests,  and  the  progress  of  depression  by  the  'strata 
of  sand  and  mud  lying  between  them.  In  short,  the  evi- 
dence would  be  strictly  on  a  parallel  with  that  furnished  by 
a  succession  of  raised  beaches  as  to  a  former  protracted  in- 
termittent elevation. 

Along  the  coasts  of  Holland  and  the  north  of  France, 
submerged  beds  of  peat  have  been  regarded  as  proofs  of 
submergence  during  historic  times.  The  amount  of  change 
varies  considerably  in  different  places,  and  here  and  there 
can  hardly  be  appreciated.  The  sinking  during  the  350 
years  preceding  1850  is  estimated  to  have  amounted  in  the 
polders  of  Grroningen  to  a  mean  annual  rate  of  8  millime- 
tres.218 In  the  north  of  France  numerous  examples  of  sub- 
merged forests  have  been  observed.  In  1846,  in  digging  the 
harbor  of  St.  Servan,  near  St.  Malo,  a  Gaulish  cemetery 
containing  ornaments  and  coins,  and  resting  on  a  still  more 
ancient  prehistoric  cemetery,  was  met  with  at  a  level  of  6 
metres  below  the  level  of  high  tide,  so  that  the  submergence 
must  have  been  at  least  to  that  extent."0 

Coral-islands.- — Evidence  of  widespread  depression, 
over  the  area  of  the  Pacific  and  Indian  Oceans,  has  been 
adduced  from  the  structure  and  growth  of  coral-reefs  and 
islands.  Mr.  Darwin,  many  years  ago,  stated  his  belief 
that,  as  the  reef-building  corals  do  not  live  at  deaths  of 
more  than  20  to  30  fathoms,  and  yet  their  reefs  rise  out 
of  deep  water,  the  sites  on  which  they  have  formed  these 
structures  must  have  subsided,  the  rate  of  subsidence  being 


p.  447.  Geol.  Mag.  vi.  p.  76;  vii.  -p.  64;  iii.  2d  ser.  p.  491;  vi.  pp.  80,  251. 
Mr.  D.  Pidgeon  has  argued  in  favor  of  the  submerged  forest  of  Torbay  having 
been  formed  without  subsidence  of  the  land.  Quart.  Journ.  Geol.  Soc.  xli. 
(1885),  p.  9.  See  also  W.  Shone,  op.  cit.  xlviii  (1892),  p.  96. 

219  Lorie,  Archives  du  Musee  Teyler,  ser.  ii.  vol.  iii.  Part  5  (1890),  p.  421. 

'm  Lorie,  ibid.  p.  438,  and  papers  cited  postea,  p.  494.  But  see  Suess, 
"Antlitz  der  Erde,"  ii.  p.  547. 


492  TEXT-BOOK    OF   GEOLOGY 

so  slow  that  the  upward  growth  of  the  reefs  has  on  the 
•whole  kept  pace  with  it."1  More  recent  researches,  how- 
ever, show  that  the  phenomena  of  coral-reefs  are  in 
some  cases,  at  least,  capable  of  satisfactory  explanation 
without  subsidence,  and  hence  that  their  existence  can 
no  longer  be  adduced  by  itself  as  a  demonstration  of  the 
subsidence  of  large  areas  of  the  ocean."2  4  The  formation  of 
coral-reefs  is  described  in  Book  III.  Part  II.  Section  iii., 
and  Mr.  Darwin's  theory  is  there  more  fully  explained. 

Distribution  of  plants  and  animals. — Since 
the  appearance  of  Edward  Forbes's  essay  upon  the  connec- 
tion between  the  distribution  of  the  existing  fauna  and  flora 
of  the  British  Isles,  and  the  geological  changes  which  have 
affected  that  area,"3  much  attention  has  been  given  to  the 
evidence  furnished  by  the  geographical  distribution  of  plants 
and  animals  as  to  geological  revolutions.  In  some  cases, 
the  former  existence  of  land  now  submerged  has  been  in- 
ferred with  considerable  confidence  from  the  distribution  of 
living  organisms,  although,  as  Mr.  Wallace  has  shown  in  the 
case  of  the  supposed  "Lemuria, "  some  of  the  inferences 
have  been  unfounded  and  unnecessary."4  The  present  dis- 
tribution of  plants  and  animals  is  only  intelligible  in  the 
light  of  former  geological  changes.  As  a  single  illustration 
of  the  kind  of  reasoning  from  present  zoological  groupings 
as  to  former  geological  subsidence,  reference  may  be  made 
to  the  fact,  that  while  the  fishes  and  mollusks  living  in  the 
seas  on  the  two  sides  of  the  Isthmus  of  Panama  are  on  the 
whole  very  distinct,  a  few  shells  and  a  large  number  of  fishes 
are  identical;  whence  the  inference  has  been  drawn  that 
though  a  broad  water-channel  originally  separated  North 
and  South  America  in  Miocene  times,  a  series  of  elevations 
and  subsidences  has  since  occurred,  the  most  recent  submer? 
sion  having  lasted  but  a  short  time,  allowing  the  passage  of 
locomotive  fishes,  yet  not  admitting  of  much  change  in  the 
comparatively  stationary  mollusks.2" 


321  See  Darwin's  "Coral  Islands,"  Dana's  "Corals  and  Coral  Islands," 
and  the  works  cited  postea,  Book  III.  Part  II.  Section  iii.  §  3,  under  "Coral- 
reefs."  The  various  theories  on  the  subject  are  discussed  by  R.  Langenbeck 
in  his  "Theorien  viber  die  Entstehung  dec  Koralleninseln  und  Korallennffe," 
1890. 

224  See  Proc.  Roy.  Phys.  Soc.  Edinburgh,  viii.  p.  1. 
223  Mem.  Geol.  Survey,  vol.  i.  1846,  p.  336. 

"24  "Island  Life,"  1880,  p.  394.  In  this  work  the  question  of  disiribution 
in  its  geological  relations  is  treated  with  admirable  lucidity  and  fulness. 

225  A.  R.  "Wallace,  "Geographical  Distribution  of  Animals,"  i.  pp.  40,  76. 


DYNAMICAL    GEOLOGY  493 

Fjords. — An  interesting  proof  of  an  extensive  depres- 
sion of  the  northwest  of  Europe  is  furnished  by  the  fjords 
or  sea-locks  by  which  that  region  is  indented.  A  fjord  is  a 
long,  narrow,  and  often  singularly  deep  inlet  of  the  sea, 
which  terminates  inland,  at  the  mouth  of  a  glen  or  valley. 
The  word  is  Norwegian,  and  in  Norway  fjords  are  charac- 
teristically developed.  The  English  word  "firth,"  however, 
is  the  same,  and  the  western  coasts  of  the  British  Isles  fur- 
nish many  excellent  examples  of  fjords,  such  as  the  Scottish 
Loch  Hourn,  Loch  Nevis,  Loch  Fyne,  Gareloch;  and  the 
Irish  Lough  Foyle,  Lough  Swilly,  Bantry  Bay,  Dunmanus 
Bay.  Similar  indentations  abound  on  the  west  coast  of  Brit- 
ish North  America  and  of  the  South  Island  of  New  Zealand. 
Some  of  the  Alpine  lakes  (Lucerne,  Grarda,  Maggiore,  and 
others),  as  well  as  many  in  Britain,  are  inland  examples  of 
fjords. 

There  can  be  little  doubt  that,  though  now  filled  with 
salt  water,  fjords  have  been  originally  land -valleys.  The 
long  inlet  was  first  excavated  as  a  valley  or  glen.  The  ad- 
jacent valley  exactly  corresponds  in  form  and  character  with 
the  hollow  of  the  fjord,  and  must  be  regarded  as  merely  its 
inland  prolongation.  That  the  glens  have  been  excavated 
by  subaerial  agents  is  a  conclusion  borne  out  by  a  great 
weight  of  evidence,  which  will  be  detailed  in  later  parts  of 
this  work.  If,  therefore,  we  admit  the  subaerial  origin  of 
the  glen,  we  must  also  grant  a  similar  origin  to  its  seaward 
prolongation.  Every  fjord  will  thus  mark  the  site  of  a  sub- 
merged valley.  This  inference  is  confirmed  by  the  fact  that 
fjords  do  not,  as  a  rule,  occur  singly,  but,  like  glens  on  land, 
lie  in  groups;  so  that,  when  found  intersecting  a  long  line 
of  coast,  such  as  that  of  the  west  of  Norway  or  the  west  of 
Scotland,  they  show  that  the  sea  now  runs  far  up  and  fills 
submerged  glens. 

Human  constructions  and  historical  rec- 
ords.—  Should  the  sea  be  observed  to  rise  to  the  level  of 
roads  and  buildings  which  it  never  used  to  touch,  should 
former  half-tide  rocks  cease  to  be  visible  even  at  low  water, 
and  should  rocks,  previously  above  the  reach  of  the  highest 
tide,  be  turned  first  into  shore-reefs,  then  into  skerries  and 
islets,  we  infer  that  the  coast-line  is  sinking.  Such  kind  of 
evidence  is  found  in  Scania,  the  most  southerly  part  of 
Sweden.  Streets,  built  of  course  above  high-water  mark, 
now  lie  below  it,  with  older  streets  lying  beneath  them,  so 
that  the  subsidence  is  of  some  antiquity.  A  stone,  the  posi- 
tion of  which  had  been  exactly  determined  by  Linnaeus  in 


494  TEXT-BOOK    OF   GEOLOGY 

1749,  was  found  after  87  years  to  be  100  feet  nearer  the 
water's  edge.226  The  west  coast  of  Greenland,  for  a  space 
of  more  than  600  miles,  is  perceptibly  sinking.  It  has  there 
been  noticed  that,  over  ancient  buildings  on  low  shores,  as 
well  as  over  entire  islets,  the  sea  has  risen.  The  Moravian 
settlers  have  been  more  than  once  driven  to  shift  their  boat- 
poles  inland,  some  of  the  old  poles  remaining  visible  under 
water.*"  Historical  evidence  likewise  exists  of  the  subsi- 
dence of  ground  in  Holland  and  Belgium.288  On  the  coast 
of  Dalmatia,  Roman  roads  and  villas  are  said  to  be  visible 
below  the  sea.229 


§  3.  Causes  of  Upheaval  and  Depression  of  Land,— These 
movements  must  again  be  traced  back  mainly  to  conse- 
quences of  the  internal  heat  of  the  earth.  There  are  various 
ways  in  which  this  cause  may  have  acted.  As  rocks  expand 
when  heated,  and  contract  on  cooling,  we  may  suppose  that, 
if  the  crust  underneath  a  tract  of  land  has  its  temperature 
slowly  raised,  as  no  doubt  takes  place  round  areas  of  nascent 
volcanoes,  a  gradual  uprise  of  the  ground  above  will  be  the 
result.  The  gradual  transference  of  the  heat  to  another 
quarter  may  produce  a  steady  subsidence.  Basing  on  the 


826  According  to  Erdmann,  the  subsidence  has  now  ceased,  or  has  even  been 
exchanged  for  an  upward  movement  (Geol.  For.  Stockholm  Forhandl.  i.  p.  93). 
Nathorst  also  thinks  that  Scania  is  now  sharing  in  the  general  elevation  of 
Scandinavia  (ibid.  p.  281).  It  appears  that  the  zero  of  movement  now  passes 
through  Bornholm  and  Laaland. 

821  These  observations,  which  have  been  accepted  for  at  least  a  generation 
past  (Proc.  Geol.  Soc.  ii.  1835,  p.  208),  have  recently  been  called  in  question, 
but  the  alleged  disproof  is  not  convincing,  and  they  are  here  retained  as  wor- 
thy of  credence.  See  Suess,  Verhand.  Geol.  Reichsanstalt,  1880,  No.  11,  and 
"Antlitz  der  Erde,"  ii.  p.  415  et  seq. 

228  Besides  the  paper  of  Lorie,  quoted  on  p.  491,  consult  Lavaleye,  "Affais- 
sement  du  sol  et  envasement  des  fleuves,  survenus  dans  les  temps  historiqties, " 
Brussels,   1859.     Grad,  Bull.   Soc.   Geol.   France,    ii.   3d  ser.  p.  46.     Arends, 
"Physische  Geschichte  der  Nordseekiiate,"  1833.     Compare  also  R.  A.  Pea- 
cock on  "Physical  and  Historical  Evidences  of  vast  Sinkings  of  land  on  the 
North  and  "West  Coasts  of  France,"  etc.,  London,  1868.     For  submerged  peat- 
beds  on  French  coast,  see  A.  Gaspard,  Ann.  Soc.  Geol.  Nord,  1870-74,  p.  40. 
On  oscillations  of  French  coast,  T.  Girard,  Bull.  Soc.  Geograph.     Paris,  ser.  6, 
vol.  x.  p.  225;  E.  Delfortrie,  Act.  Soc.  Linn.  Bordeaux,  ser.  4,  vol.  i.  p.  79. 

229  Boll.  Com.  Geol.  Italiauo,  1S74,  p.  57. 


DYNAMICAL    GEOLOGY  495 

calculations  of  Colonel  Totten,  cited  on  p.  508,  Lyell  esti- 
mated that  a  mass  of  red  sandstone  one  mile  thick,  having 
its  temperature  augmented  200°  Fahr.,  would  raise  the  over- 
lying rocks  10  feet,  and  that  a  portion  of  the  earth's  crust 
of  similar  character  50  miles  thick,  -with  an  increase  of  600° 
or  800°,  might  produce  an  elevation  of  1000  or  1500  feet.m 
But  this  computation,  as  Mr.  Mellard  Eeade  has  pointed 
out,  takes  account  only  of  linear  expansion.  If  from  any 
cause  the  mass  of  rock  whose  temperature  was  augmented 
could  not  expand  horizontally  it  would  rise  vertically,  and 
unless  some  of  the  surplus  volume  could  be  disposed  of  by 
condensation  of  the  rock,  the  uprise  would  be  three  times  as 
much  as  the  linear  extension.  Taking  this  view  of  the  case, 
we  find  that  a  mass  of  the  earth's  crust  twenty  miles  thick, 
heated  1000°  Fahr.,  and  prevented  from  extending  laterally, 
would  rise  1650  feet.881 

Again,  rocks  expand  by  fusion  and  contract  on  solidifi- 
cation. Hence,  by  the  alternate  melting  and  solidifying  of 
subterranean  masses,  upheaval  and  depression  of  the  surface 
may  possibly  be  produced  (see  pp.  508,  516). 

But  evidently  processes  of  this  nature  can  only  effect 
changes  of  level  limited  in  amount  and  local  in  area.  When 
we  consider  the  wide  tracts  over  which  terrestrial  move- 
ments are  now  taking  place,  or  have  occurred  in  past  time, 
the  explanation  of  them  must  manifestly  be  sought  in  some 
far  more  widespread  and  generally  effective  force  in  geo- 
logical dynamics.  It  must  be  confessed,  however,  that  no 
altogether  satisfactory  solution  of  the  problem  has  yet  been 
given,  and  that  the  subject  still  remains  beset  with  many 
difficulties. 


•230  "Principles,"  u.  p.  235. 

*31  Mellard  Reade,  "Origin  of  Mountain  Ranges"  (1886),  pp.  112,  114. 


496  TEXT-BOOK    OF   GEOLOGY 

Professor  Darwin,  in  one  of  his  memoirs  already  cited 
(ante,  p.  46),  has  suggested  a  possible  determining  cause  of 
the  larger  features  of  the  earth's  surface.  Assuming  for 
his  theory  a  certain  degree  of  viscosity  in  the  earth,  he 
points  out  that,  under  the  combined  influence  of  rotation 
and  the  moon's  attraction,  the  polar  regions  tend  to  outstrip 
the  equator,  and  to  acquire  a  consequent  slow  motion  from 
west  to  east  relatively  to  the  equator.  The  amount  of  dis- 
tortion produced  by  this  screwing  motion  he  finds  to  have 
been  so  slow,  that  45,000,000  years  ago,  a  point  in  lat.  30° 
would  have  been  4f,  and  a  point  in  lat.  60°  14J'  further 
west,  with  reference  to  the  equator,  than  they  are  at 
present.  This  slight  transference  shows  us,  he  remarks, 
that  the  amount  of  distortion  of  the  surface  strata  from 
this  cause  must  be  exceedingly  minute.  But  it  is  conceiv- 
able that,  in  earlier  conditions  of  the  planet,  this  screwing 
action  of  the  earth  may  have  had  some  influence  in  deter- 
mining the  surface  features  of  the  planet.  In  a  body  not 
perfectly  homogeneous  it  might  originate  wrinkles  at  the 
surface  running  perpendicular  to  the  direction  of  greatest 
pressure.  "In  the  case  of  the  earth,  the  wrinkles  would 
run  north  and  south  at  the  equator,  and  would  bear  away 
to  the  eastward  in  northerly  and  southerly  latitudes,  so 
that  at  the  north  pole  the  trend  would  be  northeast,  and 
at  the  south  pole  northwest.  Also  the  intensity  of  the 
wrinkling  force  varies  as  the  square  of  the  cosine  of  the 
latitude,  and  is  thus  greatest  at  the  equator  and  zero  at 
the  poles.  Any  wrinkle,  when  once  formed,  would  have 
a  tendency  to  turn  slightly,  so  as  to  become  more  nearly 
east  and  west  than  it  was  when  first  made." 

According  to  the  theory,  the  highest  elevations  of  the 
earth's  surface  should  be  equatorial,  and  should  have  a  gen- 


DYNAMICAL    GEOLOGY  497 

eral  north  and  south  trend,  while  in  the  northern  hemi- 
sphere the  main  direction  of  the  masses  of  land  should 
bend  round  toward  northeast,  and  in  the  opposite  hemi- 
sphere toward  southeast.  Prof.  Darwin  thinks  that  the 
general  facts  of  terrestrial  geography  tend  to  corroborate 
his  theoretical  views,  though  he  admits  that  some  are  very 
unfavorable  to  them.  In  the  discussion  of  such  a  theory, 
however,  we  must  remember  that  the  present  mountain- 
chains  on  the  earth's  surface  are  not  aboriginal,  but  arose 
at  many  successive  and  widely-separated  epochs.  Now  it 
is  quite  certain  that  the  younger  mountain-chains  (and 
these  include  the  loftiest  on  the  surface  of  the  globe) 
arose,  or  at  least  received  their  chief  upheaval,  (luring  the 
Tertiary  periods — a  comparatively  late  date  in  geological 
history.  Unless  we  are  to  enlarge  enormously  the  limits 
of  time  which  physicists  are  willing  to  concede  for  the 
evolution  of  the  whole  of  that  history,  we  can  hardly  sup- 
pose that  the  elevation  of  the  great  mountain-chains  took 
place  at  an  epoch  at  all  approaching  an  antiquity  of  45,000,- 
000  years.  Yet,  according  to  Prof.  Darwin's  showing,  the 
superficial  effects  of  internal  distortion  must  have  been  ex- 
ceedingly minute  during  the  past  45,000,000  years.  We 
must  either  therefore  multiply  enormously  the  periods  re- 
quired for  geological  changes,  or  find  some  cause  which 
could  have  elevated  great  mountain-chains  at  more  recent 
intervals. 

But  it  is  well  worth  consideration  whether  the  cause 
suggested  by  Prof.  Darwin  may  not  have  given  their  initial 
trend  to  the  masses  of  land,  so  that  any  subsequent  wrink- 
ling of  the  terrestrial  surface,  due  to  any  other  cause,  would 
be  apt  to  take  place  along  the  original  lines.  To  be  able 
to  answer  this  question,  »  it  is  necessary  to  ascertain  the 


498  TEXT- BOOK  OF  GEOLOGY 

dominant  line  of  strike  of  the  older  geological  formations. 
But  information  on  this  subject  is  still  scanty.  In  Western 
Europe,  the  prevalent  line  along  which  terrestrial  plications 
took  place  during  Palaeozoic  time  was  certainly  from  S.W. 
or  S.S.W.  to  N.B.  or  N.N.E.,  and  the  same  direction  is 
recognizable  in  the  eastern  States  of  North  America.  But 
the  trend  of  later  formations  is  more  varied.  The  striking 
contradictions  between  the  actual  direction  of  so  many 
mountain-chains  and  masses  of  land,  and  what  ought  to 
be  their  line  according  to  the  theory,  seem  to  indicate  that 
while  the  effects  of  internal  distortion  may  have  given  the 
first  outlines  to  the  land-areas  of  the  globe,  some  other  cause 
has  been  at  work  in  later  times,  acting  sometimes  along  the 
original  lines,  sometimes  across  them. 

The  main  cause  to  which. geologists  are  now  disposed 
to  refer  the  corrugations  of  the  earth's  surface  is  secular 
cooling  and  consequent  contraction."3  If  our  planet  has 
been  steadily  losing  heat  by  radiation  into  space,  it  must 
have  progressively  diminished  in  volume.  The  cooling 
implies  contraction.  According  to  Mallet,  the  diameter 
of  the  earth  is  less  by  at  least  189  miles  since  the  time 
when  the  planet  was  a  mass  of  liquid."3  But  the  contrac- 
tion has  not  manifested  itself  uniformly  over  the  whole 
surface  of  the  planet.  The  crust  varies  much  in  structure, 
in  thermal  resistance,  and  in  the  position  of  its  isogeother- 
mal  lines.  As  the  hotter  nucleus  contracts  more  rapidly 
by  cooling  than  the  cooled  and  hardened  crust,  the  latter 
must  sink  down  by  its  own  weight,  and  in  so  doing  re- 
quires to  accommodate  itself  to  a  continually  diminishing 


539  For  an  able  criticism  of  this  view  see  Fisher's  "Physics  of  Earth's  Crust,' 
2d  Edit.    Consult  also  Mr.  Reade's  "Origin  of  Mountain  Ranges. " 
233  Phil.  Trans.  1873,  p.  205. 


DYNAMICAL    GEOLOGY  499 

diameter.  The  descent  of  the  crust  gives  rise  to  enormous 
tangential  pressures.  The  rocks  are  crushed,  crumpled,  and 
broken  in  many  places.  Subsidence  must  have  been  the 
general  rule,  but  every  subsidence  would  doubtless  be  ac- 
companied with  upheavals  of  a  more  limited  kind.  The 
direction  of  these  upheaved  tracts,  whether  determined,  as 
Prof.  Darwin  suggests,  by  the  effects  of  internal  distortion, 
or  by  some  original  features  in  the  structure  of  the  crust, 
would  be  apt  to  be  linear.  The  lines,  once  taken  as  lines 
of  weakness  or  relief  from  the  intense  strain,  would  prob- 
ably be  made  use  of  again  and  again  at  successive  parox- 
ysms or  more  tranquil  periods  of  contraction.  Mallet 
ingeniously  connected  these  movements  with  the  linear 
direction  of  mountain-chains,  volcanic  vents,  and  earth- 
quake shocks.  If  the  initial  trend  to  the  land-masses  were 
given  as  hypothetically  stated  by  Prof.  Darwin,  we  may 
conceive  that  after  the  outer  parts  of  the  globe  had  attained 
a  considerable  rigidity  and  could  then  be  only  slightly  in- 
fluenced by  internal  distortion,  the  effects  of  continued 
secular  contraction  would  be  seen  in  the  intermittent  sub- 
sidence of  the  oceanic  basins  already  existing,  and  in  the 
successive  crumpling  and  elevation  of  the  intervening  stiff- 
ened terrestrial  ridges. 

This  view,  variously  modified,  has  been  widely  accepted 
by  geologists  as  furnishing  an  explanation  of  the  origin  of 
the  upheavals  and  subsidences  of  which  the  earth's  crust 
contains  such  a  long  record.  But  it  is  not  unattended  with 
objections.  The  difficulty  of  conceiving  that  a  globe  pos- 
sessing on  the  whole  a  rigidity  equal  to  that  of  glass  or 
steel  could  be  corrugated  as  the  crust  of  the  earth  has  been, 
has  led  some  writers  to  adopt  the  hypothesis  already  de- 
scribed (ante,  p.  105),  of  an  intermediate  viscous  layer  be- 


500  TEXT-BOOK    OF   GEOLOGY 

tween  the  solid  crust  and  the  solid  nucleus,  while  others 
have  suggested  that  the  observed  subsidence  may  have 
been  caused,  or  at  least  aggravated,  by  the  escape  of  vapors 
from  volcanic  orifices.  But  with  modifications,  the  main 
cause  of  terrestrial  movements  is  still  sought  in  secular 
contraction. 

Some  observers,  following  an  original  suggestion  of 
Babbage,"4  have  supposed  that  upheaval  and  subsidence, 
together  with  the  solidification,  crystallization,  and  meta- 
morphism  of  the  layers  of  the  earth's  crust,  may  have  been 
in  large  measure  due  to  the  deposition  and  removal  of 
mineral  matter  on  the  surface.  There  can  be  no  doubt 
that  the  lines  of  equal  internal  temperature  (isogeothermal 
lines)  for  a  considerable  depth  downward,  follow  approxi- 
mately the  contours  of  the  surface,  curving  up  and  down  as 
the  surface  rises  into  mountains  or  sinks  into  plains.  The 
deposition  of  a  thousand  feet  of  rock  will,  of  course,  cause 
a  corresponding  rise  in  the  isogeotherms,  and  if  we  assume 
the  average  rise  of  temperature  to  be  1°  Fahr.  for  every 
50  feet,  then  the  temperature  of  the  crust  immediately 
below  this  deposited  mass  of  rock  will  be  raised  20°.  But 
masses  of  sediment  of  much  greater  thickness  have  been 
laid  down,  and  we  may  admit  that  a  much  greater  increase 
of  temperature  than  20°  has  been  effected  by  this  means. 
On  the  other  hand,  the  denudation  of  the  land  must  lead 
to  a  depression  of  the  isogeotherms,  and  a  consequent  cool- 
ing of  the  upper  layers  of  the  crust. 

It  may  be  conceded  that  in  so  far  as  the  internal  struc- 
ture of  rocks  may  be  modified  by  such  progressive  increase 
of  temperature  as  would  arise  from  superficial  deposit,  this 


234  Journ.  Geol.  Soc.  iii.  (1834),  p.  206. 


DYNAMICAL    GEOLOGY  501 

cause  of  change  must  have  a  place  in  geological  dynamics. 
But  it  has  been  urged  that  besides  this  effect,  the  removal 
of  rock  by  denudation  from  one  area  and  its  accumulation 
upon  another  affects  the  equilibrium  of  the  crust;  that  the 
portions  where  denudation  is  active,  being  relieved  of 
weight,  rise,  while  those  where  deposition  is  prolonged, 
being  on  the  contrary  loaded,  sink.*"  This  hypothesis 
has  recently  been  strongly  advocated  by  some  of  the  geolo- 
gists who  have  been  exploring  the  Western  Territories  of 
America,  and  who  point  in  proof  of  its  truth  to  evidence 
of  continuous  subsidence  in  tracts  where  there  was  pro- 
longed deposition,  and  of  the  uprise  and  curvature  of 
originally  horizontal  strata  over  mountain  ranges  like  the 
Uinta  Mountains  in  Wyoming  and  Utah,  which  have  been 
for  a  long  time  out  of  water.  To  suppose,  however,  that 
the  removal  and  deposit  of  a  few  thousand  feet  of  rock 
should  so  seriously  affect  the  equilibrium  of  the  crust  as 
to  cause  it  to  sink  and  rise  in  proportion,  would  evince 
such  a  mobility  in  the  earth  as  could  not  fail  to  manifest 
itself  in  a  far  more  powerful  way  under  the  influence  of 
lunar  and  solar  attraction.  That  there  has  always  been  the 
closest  relation  between  upheaval  and  denudation  on  the 
one  hand,  and  subsidence  and  deposition  on  the  other,  is 
undoubtedly  true.  But  denudation  has  been  one  of  the 
consequences  of  upheaval,  and  deposition  has  been  kept 
up  only  by  continual  subsidence. 

We  are  concerned  in  the  present  part  of  this  work  only 
with  the  surface  features   of   the   land   in   so   far  as   they 


235  Similarly  it  has  been  contended  that  the  accumulation  of  a  massive  ice- 
sheel  on  the  land  would  cause  a  depression  of  the  terrestrial  surface.  N.  S. 
Shaler,  Proc.  Boston  Nat.  Hist.  Soc.  xvii.  p.  288.  T.  F.  Jamieson,  Quart. 
Journ.  Geol.  Soc.  1882,  and  Geol.  Mag.  1882,  pp.  400,  526.  Fisher,  "Physics 
of  Earth's  Crust,"  p.  223. 


502  TEXT-BOOK    OF   GEOLOGY 

bear  on  questions  of  geological  dynamics.  The  history  of 
these  features  will  be  more  conveniently  treated  in  Book 
VII.  after  the  structure  and  history  of  the  crust  have  been 
described.  Before  quitting  the  subject,  however,  we  may 
observe  that  the  larger  terrestrial  features,  such  as  the 
great  ocean  basins,  the  lines  of  submarine  ridge  surmounted 
here  and  there  by  islands  chiefly  of  volcanic  materials,  the 
continental  masses  of  land,  and  at  least  the  cores  of  most 
great  mountain-chains,  are  in  the  main  of  high  antiquity, 
stamped  as  it  were  from  the  earliest  geological  ages  on  the 
physiognomy  of  the  globe,  and  that  their  present  aspect  has 
been  the  result  not  merely  of  original  hypogene  operations, 
but  of  long-continued  superficial  action  by  the  epigene 
forces  described  in  Book  111.  Part  II. 


END   OP   FIRST   PART   OP    "TEXT-BOOK   OF   GEOLOGf" 


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